* provider/google-cloud: Add maintenance window
Allows specification of the `maintenance_window` within the `settings`
block. This controls when Google will restart a database in order to
apply updates. It is also possible to select an `update_track` to
relatively control updating between instances in the same project.
* Adjustments as suggested in code review.
Cloud SQL Gen 2 instances come with a default 'root'@'%' user on
creation. This change automatically deletes that user after creation. A
Terraform user must use the google_sql_user to create a user with
appropriate host and password.
