test(functional): Use direct WSGI requests in lieu of a wsgiref server

The wsgiref server is very flaky when running in the gate, so let's just
pass WSGI requests directly to the app for now, at look into adding a
devstack job to the gate that will use a standalone server.

Change-Id: Ifb6cd0c9080ded7ab93cced6e0d40a776eb2cd50
Merge "test(functional): Use direct WSGI requests in lieu of a wsgiref server"