cls/rbd: renamed image group add/remove/get operations

cls/rbd: renamed image group add/remove/get operations

The methods are used by the luminous release of librbd but are essentially
no-ops since group support was removed from the API. However, we need to
prevent odd interactions once a new client creates a group and uses an
older client to manipulate the image.

Signed-off-by: Jason Dillaman <dillaman@redhat.com>