Catches 401 HTTPUnauthorized and does re-login.

This fixes Midonet issue #656 (now closed, https://github.com/midokura/midonet/issues/656) and Python-Midonetclient #23 (https://github.com/midokura/python-midonetclient/issues/23). It extends Python Midonet Client to catch a 401 response sent from Keystone via Midonet API when the authentication token expires and to issue a re-login request.