* Add support for maintenance window on google_container_cluster (#526)
* Address review comments
- Set ForceNew: true on the schema element daily_maintenance_window
- Correct resource name in acceptance test
- Correct documentation of resource attribute maintenance_policy.0.daily_maintenance_window.0.duration
* Used standard validation functions where possible, added a GCP name validation function.
* Add tests for GCP name, factor out a ValidateRegexp function.
* make fmt