Add support for HTTP proxies to allow for debugging requests

[dekobon]

If it was easy to hook up the gateway to a HTTP proxy utility then we would be able to debug authentication issues more effectively. Currently, a proxy can be added manually by editing the oss/etc/nginx/templates/upstreams.conf.template file and then hardcoding in the proxy server and port. The host continues to map to the correct value because it is coded elsewhere in the configuration.