provider/aws: Support Import aws_rds_cluster (#7366)

```
make testacc TEST=./builtin/providers/aws
TESTARGS='-run=TestAccAWSRDSCluster_importBasic'
==> Checking that code complies with gofmt requirements...
go generate $(go list ./... | grep -v /vendor/)
TF_ACC=1 go test ./builtin/providers/aws -v
-run=TestAccAWSRDSCluster_importBasic -timeout 120m
=== RUN   TestAccAWSRDSCluster_importBasic
--- FAIL: TestAccAWSRDSCluster_importBasic (122.71s)
    testing.go:255: Step 1 error: ImportStateVerify attributes not
    equivalent. Difference is shown below. Top is actual, bottom is
    expected.

    (map[string]string) {

    }

(map[string]string) (len=1) {
         (string) (len=19) "skip_final_snapshot": (string) (len=4) "true"

}

FAIL
exit status 1
FAIL    github.com/hashicorp/terraform/builtin/providers/aws    122.733s
make: *** [testacc] Error 1
```

builtin/providers/aws/import_aws_rds_cluster_test.go

@@ -0,0 +1,32 @@
+package aws
+
+import (
+	"testing"
+
+	"github.com/hashicorp/terraform/helper/acctest"
+	"github.com/hashicorp/terraform/helper/resource"
+)
+
+func TestAccAWSRDSCluster_importBasic(t *testing.T) {
+	resourceName := "aws_rds_cluster.default"
+	ri := acctest.RandInt()
+
+	resource.Test(t, resource.TestCase{
+		PreCheck:     func() { testAccPreCheck(t) },
+		Providers:    testAccProviders,
+		CheckDestroy: testAccCheckAWSClusterDestroy,
+		Steps: []resource.TestStep{
+			resource.TestStep{
+				Config: testAccAWSClusterConfig(ri),
+			},
+
+			resource.TestStep{
+				ResourceName:      resourceName,
+				ImportState:       true,
+				ImportStateVerify: true,
+				ImportStateVerifyIgnore: []string{
+					"master_password", "skip_final_snapshot"},
+			},
+		},
+	})
+}

builtin/providers/aws/resource_aws_rds_cluster.go

@@ -20,6 +20,9 @@ func resourceAwsRDSCluster() *schema.Resource {
 		Read:   resourceAwsRDSClusterRead,
 		Update: resourceAwsRDSClusterUpdate,
 		Delete: resourceAwsRDSClusterDelete,
+		Importer: &schema.ResourceImporter{
+			State: resourceAwsRdsClusterImport,
+		},
 
 		Schema: map[string]*schema.Schema{
 
@@ -198,6 +201,15 @@ func resourceAwsRDSCluster() *schema.Resource {
 	}
 }
 
+func resourceAwsRdsClusterImport(
+	d *schema.ResourceData, meta interface{}) ([]*schema.ResourceData, error) {
+	// Neither skip_final_snapshot nor final_snapshot_identifier can be fetched
+	// from any API call, so we need to default skip_final_snapshot to true so
+	// that final_snapshot_identifier is not required
+	d.Set("skip_final_snapshot", true)
+	return []*schema.ResourceData{d}, nil
+}
+
 func resourceAwsRDSClusterCreate(d *schema.ResourceData, meta interface{}) error {
 	conn := meta.(*AWSClient).rdsconn
 	tags := tagsFromMapRDS(d.Get("tags").(map[string]interface{}))
@@ -407,6 +419,7 @@ func resourceAwsRDSClusterRead(d *schema.ResourceData, meta interface{}) error {
 		d.Set("database_name", dbc.DatabaseName)
 	}
 
+	d.Set("cluster_identifier", dbc.DBClusterIdentifier)
 	d.Set("db_subnet_group_name", dbc.DBSubnetGroup)
 	d.Set("parameter_group_name", dbc.DBClusterParameterGroup)
 	d.Set("db_cluster_parameter_group_name", dbc.DBClusterParameterGroup)