provider/aws: Removal of Optional from aws_ses_domain_identity arn param

```
% make testacc TEST=./builtin/providers/aws TESTARGS='-run=TestAccAwsSESDomainIdentity_'
==> Checking that code complies with gofmt requirements...
go generate $(go list ./... | grep -v /terraform/vendor/)
2017/05/09 13:05:15 Generated command/internal_plugin_list.go
TF_ACC=1 go test ./builtin/providers/aws -v -run=TestAccAwsSESDomainIdentity_ -timeout 120m
=== RUN   TestAccAwsSESDomainIdentity_basic
--- PASS: TestAccAwsSESDomainIdentity_basic (23.53s)
PASS
ok  	github.com/hashicorp/terraform/builtin/providers/aws	23.545s
```

builtin/providers/aws/resource_aws_ses_domain_identity.go

@@ -19,17 +19,16 @@ func resourceAwsSesDomainIdentity() *schema.Resource {
 		},
 
 		Schema: map[string]*schema.Schema{
-			"arn": &schema.Schema{
+			"arn": {
 				Type:     schema.TypeString,
-				Optional: true,
 				Computed: true,
 			},
-			"domain": &schema.Schema{
+			"domain": {
 				Type:     schema.TypeString,
 				Required: true,
 				ForceNew: true,
 			},
-			"verification_token": &schema.Schema{
+			"verification_token": {
 				Type:     schema.TypeString,
 				Computed: true,
 			},

builtin/providers/aws/resource_aws_ses_domain_identity_test.go

@@ -23,7 +23,7 @@ func TestAccAwsSESDomainIdentity_basic(t *testing.T) {
 		Providers:    testAccProviders,
 		CheckDestroy: testAccCheckAwsSESDomainIdentityDestroy,
 		Steps: []resource.TestStep{
-			resource.TestStep{
+			{
 				Config: fmt.Sprintf(testAccAwsSESDomainIdentityConfig, domain),
 				Check: resource.ComposeTestCheckFunc(
 					testAccCheckAwsSESDomainIdentityExists("aws_ses_domain_identity.test"),