Replace assertEqual(None, *) with assertIsNone in tests

Replace assertEqual(None, *) with assertIsNone in tests to have
more clear messages in case of failure.

Change-Id: I2732b3e747bd9394f9add7f1b077b366864fddd7
Closes-Bug:#1280522
Merge "Replace assertEqual(None, *) with assertIsNone in tests"
Merge branch 'master' of https://github.com/openstack/tempest