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.
Possibly. I was thinking if it gets the header but it's from a host that isn't allowed to be proxied, it could just be spurious and it might actually be a real request from the client for a config.

I could go either way, though. If it gets a request, which isn't in the approved proxies, and the client doesn't exist in bcfg2 that it's being sent from, does that error out? Isn't that similar to receiving a request from a client that doesn't exist at all, but has the correct auth?