Modify assert statement when comparing with None

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

Also, replace assertTrue(* is not None) with assertIsNotNone
for the same reason.

Change-Id: If41a71bd750057c7136b03bef94c04517092134c
Closes-Bug: #1280522
Merge "Modify assert statement when comparing with None"