webfaf2: add configurable proxy fix

Required when running faf behind HTTP proxy to properly rewrite
environment variables

See
http://flask.pocoo.org/docs/0.10/deploying/wsgi-standalone/#proxy-setups

Signed-off-by: Richard Marko <rmarko@fedoraproject.org>
I'd prefer to have it default to `False` rather than throw an exception if the key is not found.
`PROXY_SETUP = str2bool(config.get("hub2.proxy_setup", "false"))`