description is now a required field for google_dns_managed_zone

The description field for a managed-zone is now a required field when using the Cloud API.
This commit defaults the field to use the text "Managed by Terraform" to minimize required boilerplate for Terraform users.

Ref: https://cloud.google.com/sdk/gcloud/reference/dns/managed-zones/create

resource_dns_managed_zone.go

@@ -32,6 +32,7 @@ func resourceDnsManagedZone() *schema.Resource {
 				Type:     schema.TypeString,
 				Optional: true,
 				ForceNew: true,
+				Default:  "Managed by Terraform",
 			},
 
 			"name_servers": &schema.Schema{

resource_dns_managed_zone_test.go

@@ -80,5 +80,4 @@ var testAccDnsManagedZone_basic = fmt.Sprintf(`
 resource "google_dns_managed_zone" "foobar" {
 	name = "mzone-test-%s"
 	dns_name = "terraform.test."
-	description = "Test Description"
 }`, acctest.RandString(10))