Add test case for update volume backup

Update volume backup is a new api implemented in volume v3 (since 3.9).
This patch is to add test case for this new feature.

Including:

[1] Add v3 backups_client as a library
[2] Add update backup api to v3 backups_client
[3] Add unit tests for update backup api
[4] Add test case: test_update_backup
[5] Add release note

Change-Id: Id50fcdbc41e9e170f3d467789f80c28bac01d434
8 files changed