Move wait_for_backup_status function to common.waiters

Currently, wait_for_backup_status function resides on
services/volume/base/base_backups_client.py.
I think we should move it to common.waiters module as
other waiters functions.

Partially implements blueprint consistent-service-method-names

Change-Id: Ie1d416e32c583d8e48140c4046df989cc209b84f
4 files changed