Implement refresh() for cinder objects

Implement refresh() for cinder objects

The following patch implements the refresh functionality
in cinder objects.  Instead of calling get_by_id() to get
the latest object attributes, refresh() can be called.
With this change, delete_volume() was updated to use
volume.refresh().

Change-Id: If0573f1c44c2e67c9a8cbd88dda45310a02e3272
Partial-Implements: blueprint cinder-objects