Merge wait_for_volume_status to common method

Both volumes_extensions_client and volumes_client contain the method
wait_for_volume_status, and the code is the same.
So this patch merges them into a common method.

Partially implements blueprint consistent-service-method-names

Change-Id: I1b4871922de32fcba3aeee7270b2c727f2d0e98a
3 files changed