provider/aws: Slight design change to aws_elasticache_replication_group

We should error check up front on the use of num_cache_nodes and
cluster_mode. This allows us to write a test to make sure all works as
expected

```
% make testacc TEST=./builtin/providers/aws TESTARGS='-run=TestAccAWSElasticacheReplicationGroup_clusteringAndCacheNodesCausesError'
==> Checking that code complies with gofmt requirements...
go generate $(go list ./... | grep -v /terraform/vendor/)
2017/05/09 19:04:56 Generated command/internal_plugin_list.go
TF_ACC=1 go test ./builtin/providers/aws -v -run=TestAccAWSElasticacheReplicationGroup_clusteringAndCacheNodesCausesError -timeout 120m
=== RUN   TestAccAWSElasticacheReplicationGroup_clusteringAndCacheNodesCausesError
--- PASS: TestAccAWSElasticacheReplicationGroup_clusteringAndCacheNodesCausesError (40.58s)
PASS
ok  	github.com/hashicorp/terraform/builtin/providers/aws	40.603s
```

builtin/providers/aws/resource_aws_elasticache_replication_group.go

@@ -165,8 +165,15 @@ func resourceAwsElasticacheReplicationGroupCreate(d *schema.ResourceData, meta i
 		params.SnapshotName = aws.String(v.(string))
 	}
 
-	if a, ok := d.GetOk("cluster_mode"); ok {
+	clusterMode, clusterModeOk := d.GetOk("cluster_mode")
-		clusterModeAttributes := a.(*schema.Set).List()
+	cacheClusters, cacheClustersOk := d.GetOk("number_cache_clusters")
+
+	if !clusterModeOk && !cacheClustersOk || clusterModeOk && cacheClustersOk {
+		return fmt.Errorf("Either `number_cache_clusters` or `cluster_mode` must be set")
+	}
+
+	if clusterModeOk {
+		clusterModeAttributes := clusterMode.(*schema.Set).List()
 		attributes := clusterModeAttributes[0].(map[string]interface{})
 
 		if v, ok := attributes["num_node_groups"]; ok {
@@ -176,10 +183,10 @@ func resourceAwsElasticacheReplicationGroupCreate(d *schema.ResourceData, meta i
 		if v, ok := attributes["replicas_per_node_group"]; ok {
 			params.ReplicasPerNodeGroup = aws.Int64(int64(v.(int)))
 		}
-	} else if v, ok := d.GetOk("number_cache_clusters"); ok {
+	}
-		params.NumCacheClusters = aws.Int64(int64(v.(int)))
+
-	} else {
+	if cacheClustersOk {
-		return fmt.Errorf("number_cache_clusters must be set if not using cluster_mode")
+		params.NumCacheClusters = aws.Int64(int64(cacheClusters.(int)))
 	}
 
 	resp, err := conn.CreateReplicationGroup(params)

builtin/providers/aws/resource_aws_elasticache_replication_group_test.go

@@ -2,6 +2,7 @@ package aws
 
 import (
 	"fmt"
+	"regexp"
 	"testing"
 
 	"github.com/aws/aws-sdk-go/aws"
@@ -240,14 +241,16 @@ func TestAccAWSElasticacheReplicationGroup_redisClusterInVpc2(t *testing.T) {
 
 func TestAccAWSElasticacheReplicationGroup_nativeRedisCluster(t *testing.T) {
 	var rg elasticache.ReplicationGroup
+	rInt := acctest.RandInt()
+	rName := acctest.RandString(10)
 
 	resource.Test(t, resource.TestCase{
 		PreCheck:     func() { testAccPreCheck(t) },
 		Providers:    testAccProviders,
 		CheckDestroy: testAccCheckAWSElasticacheReplicationDestroy,
 		Steps: []resource.TestStep{
-			resource.TestStep{
+			{
-				Config: testAccAWSElasticacheReplicationGroupNativeRedisClusterConfig,
+				Config: testAccAWSElasticacheReplicationGroupNativeRedisClusterConfig(rInt, rName),
 				Check: resource.ComposeTestCheckFunc(
 					testAccCheckAWSElasticacheReplicationGroupExists("aws_elasticache_replication_group.bar", &rg),
 					resource.TestCheckResourceAttr(
@@ -268,6 +271,23 @@ func TestAccAWSElasticacheReplicationGroup_nativeRedisCluster(t *testing.T) {
 	})
 }
 
+func TestAccAWSElasticacheReplicationGroup_clusteringAndCacheNodesCausesError(t *testing.T) {
+	rInt := acctest.RandInt()
+	rName := acctest.RandString(10)
+
+	resource.Test(t, resource.TestCase{
+		PreCheck:     func() { testAccPreCheck(t) },
+		Providers:    testAccProviders,
+		CheckDestroy: testAccCheckAWSElasticacheReplicationDestroy,
+		Steps: []resource.TestStep{
+			{
+				Config:      testAccAWSElasticacheReplicationGroupNativeRedisClusterErrorConfig(rInt, rName),
+				ExpectError: regexp.MustCompile("Either `number_cache_clusters` or `cluster_mode` must be set"),
+			},
+		},
+	})
+}
+
 func TestResourceAWSElastiCacheReplicationGroupIdValidation(t *testing.T) {
 	cases := []struct {
 		Value    string
@@ -754,11 +774,8 @@ resource "aws_elasticache_replication_group" "bar" {
 }
 `, acctest.RandInt(), acctest.RandInt(), acctest.RandInt(), acctest.RandInt(), acctest.RandString(10))
 
-var testAccAWSElasticacheReplicationGroupNativeRedisClusterConfig = fmt.Sprintf(`
+func testAccAWSElasticacheReplicationGroupNativeRedisClusterErrorConfig(rInt int, rName string) string {
-provider "aws" {
+	return fmt.Sprintf(`
-    region = "us-west-2"
-}
-
 resource "aws_vpc" "foo" {
     cidr_block = "192.168.0.0/16"
     tags {
@@ -818,5 +835,70 @@ resource "aws_elasticache_replication_group" "bar" {
       replicas_per_node_group = 1
       num_node_groups = 2
     }
+    number_cache_clusters = 3
+}`, rInt, rInt, rInt, rInt, rName)
+}
+
+func testAccAWSElasticacheReplicationGroupNativeRedisClusterConfig(rInt int, rName string) string {
+	return fmt.Sprintf(`
+resource "aws_vpc" "foo" {
+    cidr_block = "192.168.0.0/16"
+    tags {
+        Name = "tf-test"
+    }
+}
+
+resource "aws_subnet" "foo" {
+    vpc_id = "${aws_vpc.foo.id}"
+    cidr_block = "192.168.0.0/20"
+    availability_zone = "us-west-2a"
+    tags {
+        Name = "tf-test-%03d"
+    }
+}
+
+resource "aws_subnet" "bar" {
+    vpc_id = "${aws_vpc.foo.id}"
+    cidr_block = "192.168.16.0/20"
+    availability_zone = "us-west-2b"
+    tags {
+        Name = "tf-test-%03d"
+    }
+}
+
+resource "aws_elasticache_subnet_group" "bar" {
+    name = "tf-test-cache-subnet-%03d"
+    description = "tf-test-cache-subnet-group-descr"
+    subnet_ids = [
+        "${aws_subnet.foo.id}",
+        "${aws_subnet.bar.id}"
+    ]
+}
+
+resource "aws_security_group" "bar" {
+    name = "tf-test-security-group-%03d"
+    description = "tf-test-security-group-descr"
+    vpc_id = "${aws_vpc.foo.id}"
+    ingress {
+        from_port = -1
+        to_port = -1
+        protocol = "icmp"
+        cidr_blocks = ["0.0.0.0/0"]
+    }
+}
+
+resource "aws_elasticache_replication_group" "bar" {
+    replication_group_id = "tf-%s"
+    replication_group_description = "test description"
+    node_type = "cache.t2.micro"
+    port = 6379
+    subnet_group_name = "${aws_elasticache_subnet_group.bar.name}"
+    security_group_ids = ["${aws_security_group.bar.id}"]
+    parameter_group_name = "default.redis3.2.cluster.on"
+    automatic_failover_enabled = true
+    cluster_mode {
+      replicas_per_node_group = 1
+      num_node_groups = 2
+    }
+}`, rInt, rInt, rInt, rInt, rName)
 }
-`, acctest.RandInt(), acctest.RandInt(), acctest.RandInt(), acctest.RandInt(), acctest.RandString(10))