Following OpenStack Style Guidelines: [1]
Unit test assertions tend to give better messages for more
specific assertions. As a result, assertIsNone(...) is
preferred over assertIs(None, ...) and assertIsNotNone(...)
is preferred over assertIsNot(None, ...).
[1] http://docs.openstack.org/developer/hacking/#unit-tests-and-assertraises
Change-Id: Ib2eea3c4fba81849338a3aad538dd58a56bc3f11