Use X-Forwared-For when behind reverse-proxy

Allow the use of [X-Forwarded-For](http://en.wikipedia.org/wiki/X-Forwarded-For) if the header exists to allow reverse-proxies and loadbalancers to be set up in front of a bcfg2-server.

So far this diff only applies to the Builtin core. I could investigate applying it to the others as well, if there's interest in this PR.
Probably the best thing is to log an error and send a 4xx-series error code, e.g.:

    msg = "So proxy much error wow"
    self.logger.error(msg)
    self.send_error(400, msg)

You might prefer 401 or another error.  In the end XML-RPC isn't as neurotic about HTTP status codes as REST.