Refactor the code in heat/tests/test_volume.py

The test code in heat/tests/test_volume.py has lots of
duplicated code and logic.
Extract them into common method and reuse the methods.

Change-Id: I5053c53a3eebf5f43cddfb96ee63a1f4f8615e76
Merge "Refactor the code in heat/tests/test_volume.py"