Stop volume leaking even if an error is occurred

In some test, if an error is occurred in setUpClass(), some resources
are not deleted because of tearDownClass() is not called.
This commit fixes it.

Partially Implements: blueprint stop-leaking

Change-Id: I6839645285da4ecd59e536b6413a67f0acaf7d6c
Merge "Stop volume leaking even if an error is occurred"