[RFE] Support any WSGI container

Running a stand-alone server is not the best tactic for production. The whole openstack is moving towards just exposing a WSGI application for use in any container (e.g. apache mod_wsgi). We should support and document it as well. Devstack support should be considered.

There's also a new goal for the Openstack services to move to WSGI in Pike[1]. Use the Gerrit topic: goal-deploy-api-in-wsgi.
There's also a tracking etherpad[2] for this goal.

[1] https://governance.openstack.org/tc/goals/pike/deploy-api-in-wsgi.html
[2] https://etherpad.openstack.org/p/ptg-pike-wsgi