This will give admins a way to revoke access to an existing cluster
once a user has been granted access.
Bumped the API microversion to 1.5 for the new endpoint.
Deprecated policy certificate:get in favor of certificate:get_ca for
clarity and consistency.
Depends-On: Ie960464e45445e195e75b91e8d65a4046eb21e93
Implements: blueprint revoke-cluster-cert
Change-Id: Ief28bef3a79f212acf4166e443a96e5419fbb757