Change assertTrue(isinstance()) by optimal assert
Change assertTrue(isinstance()) by optimal assert

assertTrue(isinstance(A, B)) or assertEqual(type(A), B) in tests
should be replaced by assertIsInstance(A, B) provided by testtools.


Change-Id: I7135d3b7fe15b16c17b7581e553ce5d289b58f43
Related-bug: #1268480

klmitch/python-glanceclient,openstack/python-glanceclient,klmitch/python-glanceclient,openstack/python-glanceclient