Cleanly override config in tests

CONF.set_override() is often called in tests but CONF.clear_override()
is never. Create a override_config() method in the base TestCase class
that restores previous conf value after test, as it is done in other
OpenStack projects.

Change-Id: Id57b4502a2a37ea12047985cf2ea6c38c6f965fd
Merge "Cleanly override config in tests"