Remove Volume V1 specific test base classes

As part of mentioned BP, all volume API tests have been shared
between V1 and V2. Now BaseVolumeV1Test & BaseVolumeV1AdminTest
class can be removed from base.py.

Partially implements: blueprint cinder-v2-api-tests

Change-Id: I57916c8cb0c4bb8d7e847655f13da2b399ab7783
Merge "Remove Volume V1 specific test base classes"