enable volumes v2 snapshot 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.

This patch includes the following volumes tests:
- test_snapshot_metadata.py
- test_volumes_snapshots.py
- test_volumes_snapshots_negative.py

Partially implements: blueprint cinder-v2-api-tests

Change-Id: I5e8a8c71d8c3a3b330a60ab0ea9603f8864518bd
Merge "enable volumes v2 snapshot tests by sharing codes"