This is the first step in removing the config dependency on "project".
This change is backwards-compatible because the value for this new
attribute defaults to the value from the provider.
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
