Currently each Versioned Object needs to implement its own get_by_id,
with this patch they don't need anymore, since it will be included in
the base class CinderObject and it will work for all the objects.
This will help for other things like having a refresh method or
conditional updates in the objects.
Related-Bug: #1490944
Related-Bug: #1238093
Related-Bug: #1490946
Related-Bug: #1469659
Change-Id: I355dc8eaefed93003533ee083f74acd1315f057e