resource/aws_rds_cluster: Add multimaster to engine_mode validation

Reference: #9690

Output from acceptance testing:

```
--- PASS: TestAccAWSRDSCluster_EngineMode_Multimaster (141.13s)
```

aws/resource_aws_rds_cluster.go

@@ -143,6 +143,7 @@ func resourceAwsRDSCluster() *schema.Resource {
 				Default:  "provisioned",
 				ValidateFunc: validation.StringInSlice([]string{
 					"global",
+					"multimaster",
 					"parallelquery",
 					"provisioned",
 					"serverless",

aws/resource_aws_rds_cluster_test.go

@@ -788,6 +788,40 @@ func TestAccAWSRDSCluster_EngineMode_Global(t *testing.T) {
 	})
 }
 
+func TestAccAWSRDSCluster_EngineMode_Multimaster(t *testing.T) {
+	var dbCluster1 rds.DBCluster
+
+	rName := acctest.RandomWithPrefix("tf-acc-test")
+	resourceName := "aws_rds_cluster.test"
+
+	resource.ParallelTest(t, resource.TestCase{
+		PreCheck:     func() { testAccPreCheck(t) },
+		Providers:    testAccProviders,
+		CheckDestroy: testAccCheckAWSClusterDestroy,
+		Steps: []resource.TestStep{
+			{
+				Config: testAccAWSRDSClusterConfig_EngineMode_Multimaster(rName),
+				Check: resource.ComposeTestCheckFunc(
+					testAccCheckAWSClusterExists(resourceName, &dbCluster1),
+					resource.TestCheckResourceAttr(resourceName, "engine_mode", "multimaster"),
+				),
+			},
+			{
+				ResourceName:      resourceName,
+				ImportState:       true,
+				ImportStateVerify: true,
+				ImportStateVerifyIgnore: []string{
+					"apply_immediately",
+					"cluster_identifier_prefix",
+					"master_password",
+					"skip_final_snapshot",
+					"snapshot_identifier",
+				},
+			},
+		},
+	})
+}
+
 func TestAccAWSRDSCluster_EngineMode_ParallelQuery(t *testing.T) {
 	var dbCluster1 rds.DBCluster
 
@@ -2580,6 +2614,57 @@ resource "aws_rds_cluster" "test" {
 `, rName, engineMode)
 }
 
+func testAccAWSRDSClusterConfig_EngineMode_Multimaster(rName string) string {
+	return fmt.Sprintf(`
+data "aws_availability_zones" "available" {
+  state = "available"
+}
+
+resource "aws_vpc" "test" {
+  cidr_block = "10.0.0.0/16"
+
+  tags = {
+    Name = "tf-acc-test-rds-cluster-multimaster"
+  }
+}
+
+resource "aws_subnet" "test1" {
+  vpc_id            = "${aws_vpc.test.id}"
+  cidr_block        = "10.0.0.0/24"
+  availability_zone = "${data.aws_availability_zones.available.names[0]}"
+
+  tags = {
+    Name = "tf-acc-test-rds-cluster-multimaster"
+  }
+}
+
+resource "aws_subnet" "test2" {
+  vpc_id            = "${aws_vpc.test.id}"
+  cidr_block        = "10.0.1.0/24"
+  availability_zone = "${data.aws_availability_zones.available.names[1]}"
+
+  tags = {
+    Name = "tf-acc-test-rds-cluster-multimaster"
+  }
+}
+
+resource "aws_db_subnet_group" "test" {
+  name       = %[1]q
+  subnet_ids = ["${aws_subnet.test1.id}", "${aws_subnet.test2.id}"]
+}
+
+# multimaster requires db_subnet_group_name
+resource "aws_rds_cluster" "test" {
+  cluster_identifier   = %[1]q
+  db_subnet_group_name = "${aws_db_subnet_group.test.name}"
+  engine_mode          = "multimaster"
+  master_password      = "barbarbarbar"
+  master_username      = "foo"
+  skip_final_snapshot  = true
+}
+`, rName)
+}
+
 func testAccAWSRDSClusterConfig_GlobalClusterIdentifier(rName string) string {
 	return fmt.Sprintf(`
 resource "aws_rds_global_cluster" "test" {

website/docs/r/rds_cluster.html.markdown

@@ -75,6 +75,21 @@ resource "aws_rds_cluster" "postgresql" {
 }
 ```
 
+### Aurora Multi-Master Cluster
+
+-> More information about Aurora Multi-Master Clusters can be found in the [RDS User Guide](https://docs.aws.amazon.com/AmazonRDS/latest/AuroraUserGuide/aurora-multi-master.html).
+
+```hcl
+resource "aws_rds_cluster" "example" {
+  cluster_identifier   = "example"
+  db_subnet_group_name = "${aws_db_subnet_group.example.name}"
+  engine_mode          = "multimaster"
+  master_password      = "barbarbarbar"
+  master_username      = "foo"
+  skip_final_snapshot  = true
+}
+```
+
 ## Argument Reference
 
 For more detailed documentation about each argument, refer to
@@ -116,7 +131,7 @@ Default: A 30-minute window selected at random from an 8-hour block of time per
 * `iam_roles` - (Optional) A List of ARNs for the IAM roles to associate to the RDS Cluster.
 * `iam_database_authentication_enabled` - (Optional) Specifies whether or mappings of AWS Identity and Access Management (IAM) accounts to database accounts is enabled. Please see [AWS Documentation](https://docs.aws.amazon.com/AmazonRDS/latest/AuroraUserGuide/UsingWithRDS.IAMDBAuth.html) for availability and limitations.
 * `engine` - (Optional) The name of the database engine to be used for this DB cluster. Defaults to `aurora`. Valid Values: `aurora`, `aurora-mysql`, `aurora-postgresql`
-* `engine_mode` - (Optional) The database engine mode. Valid values: `global`, `parallelquery`, `provisioned`, `serverless`. Defaults to: `provisioned`. See the [RDS User Guide](https://docs.aws.amazon.com/AmazonRDS/latest/UserGuide/aurora-serverless.html) for limitations when using `serverless`.
+* `engine_mode` - (Optional) The database engine mode. Valid values: `global`, `multimaster`, `parallelquery`, `provisioned`, `serverless`. Defaults to: `provisioned`. See the [RDS User Guide](https://docs.aws.amazon.com/AmazonRDS/latest/UserGuide/aurora-serverless.html) for limitations when using `serverless`.
 * `engine_version` - (Optional) The database engine version. Updating this argument results in an outage. See the [Aurora MySQL](https://docs.aws.amazon.com/AmazonRDS/latest/AuroraUserGuide/AuroraMySQL.Updates.html) and [Aurora Postgres](https://docs.aws.amazon.com/AmazonRDS/latest/AuroraUserGuide/AuroraPostgreSQL.Updates.html) documentation for your configured engine to determine this value. For example with Aurora MySQL 2, a potential value for this argument is `5.7.mysql_aurora.2.03.2`.
 * `source_region` - (Optional) The source region for an encrypted replica DB cluster.
 * `enabled_cloudwatch_logs_exports` - (Optional) List of log types to export to cloudwatch. If omitted, no logs will be exported.