Fixes volume client in volume tests

volumes_client is coming from cls.os and volume_client is coming
from cls.os_adm. Here it creates a naming disaster for the volume
client. Adding admin_volume_client at the place of volume_client
fixes the client for admin user.

Change-Id: I0c55d3f7fbf4a096c012ffee3697f14869fe9962
Merge "Fixes volume client in volume tests"