Support control-plane API services deployed via WSGI

https://github.com/openstack/governance/blob/master/goals/pike/deploy-api-in-wsgi.rst
There is a pike community goal, that asks each projects that deploys and API to allow deploying it in wsgi mode. kuryr-libnetwork deploys a flask application, that can be used in wsgi mode already. however for the goal to be completed:

1) Provide WSGI application script file(s) (e.g. to be used by web server). There shouldn't be any web server restriction and the application could be deploying to any web server that support WSGI applications.
2) Switch devstack jobs to deploy control-plane API services in WSGI with Apache. Usage of Apache is already the default in Devstack, let's keep using it for consistency unless there is some efforts to support another web server but this is not the case at this time.

So we probably need to add a .wsgi file for convenience of mod_wsgi users and switch devstack to deploy everything under Apache

Gerrit topic: https://review.openstack.org/#q,topic:bp/deploy-kuryr-libnetwork-api-in-wsgi,n,z

Addressed by: https://review.openstack.org/374315
    Allow running kuryr-libnetwork with uwsgi