Modify assert statement when comparing with None

Replace assertEqual(None, *) with assertIsNone in heat's
tests to have more clear messages in case of failure.
Also, replace assertNotEqual(None, *) with assertIsNotNone
for the same reason.

Change-Id: I34dbf9b87c7c8c6ca4949b53bd5ef763a9c2fc24
Closes-Bug: #1280522
Merge "Modify assert statement when comparing with None"