Fix django tests

Set the environment in `run-tests.py` and import from the correct places for compatibility.

Setting the environment in this way does not allow for nosetests to run a single test. I think that this is an important capability, and it would be trivial to set the environment like this until we switch to the Django test runner. 

    if __name__ == __main__:
        os.environ['DJANGO_SETTINGS_MODULE'] = 'pulp.server.webservices.settings'