Add extensions listing to the controller

Add extensions listing to the controller

Extensions aren't like normal resources handled by the
core plugin or a service plugin so they need special code
to handle them. The resource identifier hook also had to
be tweaked to let the request slip through without having
an entry in the attributes map.

Change-Id: I5f42bdd7aa3c103b2ff3550ce1b0d9a3baf35a35
Co-Authored-By: Brandon Logan <brandon.logan@rackspace.com>
Partially-Implements: blueprint wsgi-pecan-switch