Use fixtures and addCleanup instead of tearDown
Use fixtures and addCleanup instead of tearDown

Nothing ensures tearDown call as tearDown is called only if test
succeeds. This change replaces tearDown use with:

* addCleanup use to stop mocks
* EnvFixture which ensures to unmock environment thanks to useFixture.

Change-Id: I1ff422e6a7585bc48b04b8f5c4cc1e7e9ddab1bc

redhat-openstack/python-openstackclient,dtroyer/python-openstackclient,redhat-openstack/python-openstackclient,openstack/python-openstackclient,openstack/python-openstackclient,dtroyer/python-openstackclient