Refer to:http://docs.openstack.org/developer/hacking/#unit-tests-and-assertraises
[H203] Use assertIs(Not)None to check for None (off by default) Unit
test assertions tend to give better messages for more specific assertions.
As a result, assertIsNone(...) is preferred over assertEqual(None, ...)
and assertIs(None, ...)
Change-Id: I9246fac952c59d3ddc1458c16c53fc988ac095d3