Snapshot and volume objects

Snapshot and volume objects

Abstract volumes and volume snapshots into objects.
Get, create, and delete snapshot APIs were changed to
use new snapshot objects.  A skeleton volume object was
created, but cinder internals were not changed to use
the volume object, although volume is referenced and
used by the snapshot object.  The internals will be
changed to use volume object in a subsequent patch.

Change-Id: I387018e80c8539565e99454db65d976030002c0f
Implements: blueprint cinder-objects