Replace assertEqual(None, *) with assertIsNone in tests

Replace assertEqual(None, *) with assertIsNone in keystone's tests
to havemore clear messages in case of failure.

Closes-Bug: #1280522
Change-Id: I5a857fcb3bfebc6ff14871df6b2cd1dee44596b2
Merge "Replace assertEqual(None, *) with assertIsNone in tests"