Backup object

Backup object

This patch adds VersionedObjects abstraction layer to volume backups.
The object derives from CinderObjectDictCompat, so it supports both
object (obj.prop) and dict (obj['prop']) syntax to access properties.
Complete move to object notation will be made in a follow up clean up
patch.

Co-Authored-By: Grzegorz Grasza <grzegorz.grasza@intel.com>
Change-Id: Icff37261b367463b71a1268be16f9c97f595bf0c
Partial-Implements: blueprint cinder-objects