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

Some of tests use different method of assertTrue(isinstance(A, B)).
The correct way is to use assertIsInstance(A,B) provided by testtools.

Change-Id: Ib04a2fd4b6545bf41b14b4366c11f4d7329227e6
Closes-bug: #1268480

openstack/python-magnumclient