Unittests: Refactor Volume resources tests

Changes include:
- use native HOT template with native Cinder resources for Cinder
  resources tests
- split tests in two test cases, one for AWS, another for native
  resources
- added base test case with most common boilerplate mocking

Change-Id: I9633a4b4eb0dc2403053ad9da6f34801604dbdce
Merge "Unittests: Refactor Volume resources tests"