Added volume backup and restore to Ceph RBD driver

It is now possible to backup and restore volumes
when using volume.drivers.rbd.RBDDriver

Implements: blueprint cinder-backup-to-ceph

Change-Id: Ic1b8db8f0acd7974423414171b8fb45197d05dc6
Merge "Added volume backup and restore to Ceph RBD driver"