Fix duplicate ref of volume_types_client in vol tests

tempest/api/volume/base.py admin base tests class has duplicate
reference for 'volume_types_client'. Tests use mix of both.

This patch fix this duplicate reference and modify tests accordingly

Closes-Bug: #1379106

Change-Id: I016d364ad792c97d1dce12ee7da225ebe0a690f6
Merge "Fix duplicate ref of volume_types_client in vol tests"