[RBD] Fix snapshot backup name
[RBD] Fix snapshot backup name

We always create a full backup for snapshots. It means we have to
generate a correct buckup name for a base backup.

Closes-Bug: #1860739
Change-Id: Ia08c252d747148e624f8d9e8b0e43f94773421e0

mahak/cinder,openstack/cinder,openstack/cinder,mahak/cinder