provider/aws: Add validation for aws_vpc.cidr_block

- closes #2340

builtin/providers/aws/resource_aws_vpc.go

@@ -3,6 +3,7 @@ package aws
 import (
 	"fmt"
 	"log"
+	"net"
 	"time"
 
 	"github.com/aws/aws-sdk-go/aws"
@@ -24,6 +25,16 @@ func resourceAwsVpc() *schema.Resource {
 				Type:     schema.TypeString,
 				Required: true,
 				ForceNew: true,
+				ValidateFunc: func(v interface{}, k string) (ws []string, errors []error) {
+					value := v.(string)
+					_, ipnet, err := net.ParseCIDR(value)
+
+					if err != nil || ipnet == nil || value != ipnet.String() {
+						errors = append(errors, fmt.Errorf(
+							"%q must contain a valid CIDR", k))
+					}
+					return
+				},
 			},
 
 			"instance_tenancy": &schema.Schema{