Add backups extension to volume API v2

Also, correct problem with URI to include
the version and tenant ID. Once this change merges,
the same change will be visible in the dev guide.

Partial-Bug: #1286075

Change-Id: I3850eb32d2ad3dffea8ea0faeddf4c215501c990
author: diane fleming
Merge "Add backups extension to volume API v2"