Reimplement Volume snapshot with backup

Use backups instead of snapshots for volume handle_snapshot: it allows a
better management of a lifecycle of the stack snapshot, because cinder
snapshots can only exist along with the original volume.

blueprint stack-snapshot
Co-Authored-By: ala.rezmerita@cloudwatt.com

Change-Id: I1ec2bd177df61cae05769628c63db0bd0790c3ce
Merge "Reimplement Volume snapshot with backup"