Use Tempest-lib's compute volumes_client

Compute volumes_client has been migrated to
tempest-lib- I10913f82dc3d0bef53edd67144cb47580d3c574d

Now Tempest-lib provides this client as stable interface so Tempest
should start using this from lib and remove its own copy.
Also remove its unit tests.

This commit makes Tempest to use compute volumes_client
from Tempest-lib.

Change-Id: I9909c96d6f7003a09f7694b1b1d7feac894b998f
5 files changed