Use unittest2 test loader when available.
Use unittest2 test loader when available.

When default setuptools test loader is used, tests are collected into
unittest.TestSuite instead of unittest2.TestSuite. As a result,
setUpClass() methods are not called and tests fail on Python 2.6.

Instead, use unittest2 test loader when available. Fallback to plain
unittest loader which is fine for newer Python versions.

coreos/mockldap