Enable some volumes v2 tests by sharing codes

1. create volumes v2 tests using the existent test class.
2. create a new test class for v1, which inherits v2.
3. add variable 'special_fields' to deal with the different
   field's name and reps code between v1 and v2.

This patch includes the following volumes tests:
- test_volume_metadata.py
- test_volume_transfers.py
- test_volumes_actions.py
- test_volumes_get.py
- test_volumes_negative.py

Partially implements: blueprint cinder-v2-api-tests

Change-Id: I30b8f38d1f8132ea28266a32d2c9cb912a70e2c3
Merge "Enable some volumes v2 tests by sharing codes"