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