provider/aws: Fix source_dest_check with network_interface

The default value for `source_dest_check` needs to remain the same, so as not to break any backwards compatibility, however, adding a new `network_interface` parameter with a pre-configured network_interface that has `source_dest_check` set to false throws a diff after initial apply. Since we don't want to change `source_dest_check` to computed in order to not break sane defaults, ignore the diff thrown if `network_interface` attributes are configured on an instance.

```
$ make testacc TEST=./builtin/providers/aws TESTARGS="-run=TestAccAWSInstance_primaryNetworkInterfaceSourceDestCheck"
==> Checking that code complies with gofmt requirements...
go generate $(go list ./... | grep -v /terraform/vendor/)
2017/04/28 16:26:02 Generated command/internal_plugin_list.go
TF_ACC=1 go test ./builtin/providers/aws -v -run=TestAccAWSInstance_primaryNetworkInterfaceSourceDestCheck -timeout 120m
=== RUN   TestAccAWSInstance_primaryNetworkInterfaceSourceDestCheck
--- PASS: TestAccAWSInstance_primaryNetworkInterfaceSourceDestCheck (134.20s)
PASS
ok      github.com/hashicorp/terraform/builtin/providers/aws    134.211s
```

```
$ make testacc TEST=./builtin/providers/aws TESTARGS="-run=TestAccAWSInstance_sourceDestCheck"
==> Checking that code complies with gofmt requirements...
go generate $(go list ./... | grep -v /terraform/vendor/)
2017/04/28 16:15:14 Generated command/internal_plugin_list.go
TF_ACC=1 go test ./builtin/providers/aws -v -run=TestAccAWSInstance_sourceDestCheck -timeout 120m
=== RUN   TestAccAWSInstance_sourceDestCheck
--- PASS: TestAccAWSInstance_sourceDestCheck (179.81s)
PASS
ok      github.com/hashicorp/terraform/builtin/providers/aws   179.815s
```

Fixes: #14068

builtin/providers/aws/resource_aws_instance.go

@@ -90,6 +90,11 @@ func resourceAwsInstance() *schema.Resource {
 				Type:     schema.TypeBool,
 				Optional: true,
 				Default:  true,
+				DiffSuppressFunc: func(k, old, new string, d *schema.ResourceData) bool {
+					// Suppress diff if network_interface is set
+					_, ok := d.GetOk("network_interface")
+					return ok
+				},
 			},
 
 			"user_data": {
@@ -642,6 +647,7 @@ func resourceAwsInstanceRead(d *schema.ResourceData, meta interface{}) error {
 		d.Set("primary_network_interface_id", primaryNetworkInterface.NetworkInterfaceId)
 		d.Set("associate_public_ip_address", primaryNetworkInterface.Association != nil)
 		d.Set("ipv6_address_count", len(primaryNetworkInterface.Ipv6Addresses))
+		d.Set("source_dest_check", *primaryNetworkInterface.SourceDestCheck)
 
 		for _, address := range primaryNetworkInterface.Ipv6Addresses {
 			ipv6Addresses = append(ipv6Addresses, *address.Ipv6Address)

builtin/providers/aws/resource_aws_instance_test.go

@@ -966,6 +966,27 @@ func TestAccAWSInstance_primaryNetworkInterface(t *testing.T) {
 	})
 }
 
+func TestAccAWSInstance_primaryNetworkInterfaceSourceDestCheck(t *testing.T) {
+	var instance ec2.Instance
+	var ini ec2.NetworkInterface
+
+	resource.Test(t, resource.TestCase{
+		PreCheck:     func() { testAccPreCheck(t) },
+		Providers:    testAccProviders,
+		CheckDestroy: testAccCheckInstanceDestroy,
+		Steps: []resource.TestStep{
+			{
+				Config: testAccInstanceConfigPrimaryNetworkInterfaceSourceDestCheck,
+				Check: resource.ComposeTestCheckFunc(
+					testAccCheckInstanceExists("aws_instance.foo", &instance),
+					testAccCheckAWSENIExists("aws_network_interface.bar", &ini),
+					resource.TestCheckResourceAttr("aws_instance.foo", "source_dest_check", "false"),
+				),
+			},
+		},
+	})
+}
+
 func TestAccAWSInstance_addSecondaryInterface(t *testing.T) {
 	var before ec2.Instance
 	var after ec2.Instance
@@ -1866,6 +1887,42 @@ resource "aws_instance" "foo" {
 }
 `
 
+const testAccInstanceConfigPrimaryNetworkInterfaceSourceDestCheck = `
+resource "aws_vpc" "foo" {
+  cidr_block = "172.16.0.0/16"
+  tags {
+    Name = "tf-instance-test"
+  }
+}
+
+resource "aws_subnet" "foo" {
+  vpc_id = "${aws_vpc.foo.id}"
+  cidr_block = "172.16.10.0/24"
+  availability_zone = "us-west-2a"
+  tags {
+    Name = "tf-instance-test"
+  }
+}
+
+resource "aws_network_interface" "bar" {
+  subnet_id = "${aws_subnet.foo.id}"
+  private_ips = ["172.16.10.100"]
+  source_dest_check = false
+  tags {
+    Name = "primary_network_interface"
+  }
+}
+
+resource "aws_instance" "foo" {
+	ami = "ami-22b9a343"
+	instance_type = "t2.micro"
+	network_interface {
+	 network_interface_id = "${aws_network_interface.bar.id}"
+	 device_index = 0
+  }
+}
+`
+
 const testAccInstanceConfigAddSecondaryNetworkInterfaceBefore = `
 resource "aws_vpc" "foo" {
   cidr_block = "172.16.0.0/16"