Use unittest2.TestSuite with py26

The tempest master branch with tox -efull with py26 command,
produces a lot of tierDownClass failures,
because it tries to tearDown an already tearDowned class.

By switching to unittest2.TestSuite the issue is solved.

Change-Id: I7a5d21af62bafb13cdc8a0dcc20ed3f742aa214f
Merge "Use unittest2.TestSuite with py26"