test_volume unittest fails if ran only this module

Using stevedore ExtensionManager to load entry_points(extensions)
before running any test case.

In BaseVolumeTestCase we have stubbed 'os.path.exists' call,
because of this stevedore is not able to load entry points paroperly.
Stevedore internally use pkg_resources and pkg_resources use
'os.path.exists' to check if entry_points.txt file exists or not.
Because of stubbibg it is returning True even if file do not exists.

Change-Id: I4fd6bee2e9f33c45ad3deacfe7359dc56171205e
Closes-Bug: #1263175
Merge "test_volume unittest fails if ran only this module"