Make CLI tests python2.6 compatible

- adds setUpClass method to tempest.test.BaseTestCase
  to provide it instead of unittest.TestCase
- adds replacement for subprocess.check_output and CalledProcessError
- also ensures closing of devnull provided to check_output

Change-Id: I90382f8553baabc592a2bf7008d591d83f89f416
Merge "Make CLI tests python2.6 compatible"