Newer Python versions don't like passing a mock in to an os.path.join:
TypeError: join() argument must be str or bytes, not 'MagicMock'
This patch also adds openstack-tox-functional-py36 to the gate queue.
Recent and major LTS distributions ship Python 3.6 or greater.
Change-Id: Ic71059da24e0d2c3f0ac3af5375906d8e8aa43f4