Define py27 assertFoo methods for py26

Frequent issue a test case is failing with py26,
because the used assert method like the assertGreaterEqual
is not defined.

Adding unittest2.TestCase to the base testclass when running
tempest with py26.

Change-Id: I5184fe88485b611c033e81f82177060460da7f1e
Merge "Define py27 assertFoo methods for py26"