Add default volume_backend_name for rbd driver in cinder volume
Add default volume_backend_name for rbd driver in cinder volume

volume_backend_name is required when using volume type.

Change-Id: Idab2ab98dea4940ba9404b219dbed935db0d51ed
Closes-Bug: #1705657

dardelean/kolla-ansible,dardelean/kolla-ansible,dardelean/kolla-ansible