provider/aws: Add support for `kms_key_id` to `aws_db_instance` (#6651)

As requested in #4822, add support for a KMS Key ID (ARN) for Db
Instance

```
make testacc TEST=./builtin/providers/aws
TESTARGS='-run=TestAccAWSDBInstance_kmsKey' 2>~/tf.log
==> Checking that code complies with gofmt requirements...
go generate $(go list ./... | grep -v /vendor/)
TF_ACC=1 go test ./builtin/providers/aws -v
-run=TestAccAWSDBInstance_kmsKey -timeout 120m
=== RUN   TestAccAWSDBInstance_basic
--- PASS: TestAccAWSDBInstance_basic (587.37s)
=== RUN   TestAccAWSDBInstance_kmsKey
--- PASS: TestAccAWSDBInstance_kmsKey (625.31s)
PASS
ok      github.com/hashicorp/terraform/builtin/providers/aws    1212.684s
```

builtin/providers/aws/resource_aws_db_instance.go

@@ -289,6 +289,13 @@ func resourceAwsDbInstance() *schema.Resource {
 				Computed: true,
 			},
 
+			"kms_key_id": &schema.Schema{
+				Type:     schema.TypeString,
+				Optional: true,
+				Computed: true,
+				ForceNew: true,
+			},
+
 			"tags": tagsSchema(),
 		},
 	}
@@ -396,6 +403,7 @@ func resourceAwsDbInstanceCreate(d *schema.ResourceData, meta interface{}) error
 
 		if attr, ok := d.GetOk("option_group_name"); ok {
 			opts.OptionGroupName = aws.String(attr.(string))
+
 		}
 
 		if attr, ok := d.GetOk("port"); ok {
@@ -492,6 +500,7 @@ func resourceAwsDbInstanceCreate(d *schema.ResourceData, meta interface{}) error
 		opts.BackupRetentionPeriod = aws.Int64(int64(attr.(int)))
 		if attr, ok := d.GetOk("multi_az"); ok {
 			opts.MultiAZ = aws.Bool(attr.(bool))
+
 		}
 
 		if attr, ok := d.GetOk("maintenance_window"); ok {
@@ -560,6 +569,10 @@ func resourceAwsDbInstanceCreate(d *schema.ResourceData, meta interface{}) error
 			opts.OptionGroupName = aws.String(attr.(string))
 		}
 
+		if attr, ok := d.GetOk("kms_key_id"); ok {
+			opts.KmsKeyId = aws.String(attr.(string))
+		}
+
 		log.Printf("[DEBUG] DB Instance create configuration: %#v", opts)
 		var err error
 		err = resource.Retry(5*time.Minute, func() *resource.RetryError {
@@ -632,6 +645,7 @@ func resourceAwsDbInstanceRead(d *schema.ResourceData, meta interface{}) error {
 	d.Set("maintenance_window", v.PreferredMaintenanceWindow)
 	d.Set("publicly_accessible", v.PubliclyAccessible)
 	d.Set("multi_az", v.MultiAZ)
+	d.Set("kms_key_id", v.KmsKeyId)
 	if v.DBSubnetGroup != nil {
 		d.Set("db_subnet_group_name", v.DBSubnetGroup.DBSubnetGroupName)
 	}

builtin/providers/aws/resource_aws_db_instance_test.go

@@ -3,6 +3,7 @@ package aws
 import (
 	"fmt"
 	"log"
+	"regexp"
 	"strings"
 
 	"math/rand"
@@ -51,6 +52,31 @@ func TestAccAWSDBInstance_basic(t *testing.T) {
 	})
 }
 
+func TestAccAWSDBInstance_kmsKey(t *testing.T) {
+	var v rds.DBInstance
+	keyRegex := regexp.MustCompile("^arn:aws:kms:")
+
+	ri := rand.New(rand.NewSource(time.Now().UnixNano())).Int()
+	config := fmt.Sprintf(testAccAWSDBInstanceConfigKmsKeyId, ri)
+
+	resource.Test(t, resource.TestCase{
+		PreCheck:     func() { testAccPreCheck(t) },
+		Providers:    testAccProviders,
+		CheckDestroy: testAccCheckAWSDBInstanceDestroy,
+		Steps: []resource.TestStep{
+			resource.TestStep{
+				Config: config,
+				Check: resource.ComposeTestCheckFunc(
+					testAccCheckAWSDBInstanceExists("aws_db_instance.bar", &v),
+					testAccCheckAWSDBInstanceAttributes(&v),
+					resource.TestMatchResourceAttr(
+						"aws_db_instance.bar", "kms_key_id", keyRegex),
+				),
+			},
+		},
+	})
+}
+
 func TestAccAWSDBInstance_optionGroup(t *testing.T) {
 	var v rds.DBInstance
 
@@ -404,6 +430,51 @@ resource "aws_db_instance" "bar" {
 	parameter_group_name = "default.mysql5.6"
 }`
 
+var testAccAWSDBInstanceConfigKmsKeyId = `
+resource "aws_kms_key" "foo" {
+    description = "Terraform acc test %s"
+    policy = <<POLICY
+{
+  "Version": "2012-10-17",
+  "Id": "kms-tf-1",
+  "Statement": [
+    {
+      "Sid": "Enable IAM User Permissions",
+      "Effect": "Allow",
+      "Principal": {
+        "AWS": "*"
+      },
+      "Action": "kms:*",
+      "Resource": "*"
+    }
+  ]
+}
+POLICY
+}
+
+resource "aws_db_instance" "bar" {
+	allocated_storage = 10
+	engine = "MySQL"
+	engine_version = "5.6.21"
+	instance_class = "db.m3.medium"
+	name = "baz"
+	password = "barbarbarbar"
+	username = "foo"
+
+
+	# Maintenance Window is stored in lower case in the API, though not strictly
+	# documented. Terraform will downcase this to match (as opposed to throw a
+	# validation error).
+	maintenance_window = "Fri:09:00-Fri:09:30"
+
+	backup_retention_period = 0
+	storage_encrypted = true
+	kms_key_id = "${aws_kms_key.foo.arn}"
+
+	parameter_group_name = "default.mysql5.6"
+}
+`
+
 var testAccAWSDBInstanceConfigWithOptionGroup = fmt.Sprintf(`
 
 resource "aws_db_option_group" "bar" {

website/source/docs/providers/aws/r/db_instance.html.markdown

@@ -102,6 +102,7 @@ database, and to use this value as the source database. This correlates to the
 enhanced monitoring metrics to CloudWatch Logs. You can find more information on the [AWS Documentation](http://docs.aws.amazon.com/AmazonRDS/latest/UserGuide/USER_Monitoring.html) 
 what IAM permissions are needed to allow Enhanced Monitoring for RDS Instances.
 * `monitoring_interval` - (Optional) The interval, in seconds, between points when Enhanced Monitoring metrics are collected for the DB instance. To disable collecting Enhanced Monitoring metrics, specify 0. The default is 0. Valid Values: 0, 1, 5, 10, 15, 30, 60.
+* `kms_key_id` - (Optional) The ARN for the KMS encryption key. 
 * `tags` - (Optional) A mapping of tags to assign to the resource.
 
 ~> **NOTE:** Removing the `replicate_source_db` attribute from an existing RDS