Sharing codes for cinder v1 and v2 tests

Tempest doesn't have enough Cinder v2 api tests. We need to add more
tests for it. Cinder v2 api only has some small updates, so v1 and v2
tests could share service client and test code.
This patch makes the follwoing initial chanages to support sharing
codes between v1 and v2:

1. Rename volumes v1 client to a base client, then create v1 and v2
   clients inheriting base client.
2. Create common base test class instead of
   BaseVolumeV1Test/BaseVolumeV2Test. They will be remove after all
   test classes using sharing codes.

Partially implements: blueprint cinder-v2-api-tests

Change-Id: I2c0139d82663563996fae702f8e5f29f7ae0a7a9
Merge "Sharing codes for cinder v1 and v2 tests"