Allow use of any backup backend for any volume backend
Allow use of any backup backend for any volume backend

Currently if Ceph is used for Cinder volumes the only choice of
backup driver is Ceph. It should be possible to use any backup
driver for any volume type, e.g. iSCSI volumes can go to Ceph
based backup storage.

Change-Id: Ifc8a767b9030391580de6a24383bb6b87053835d
Closes-Bug: #1683715

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