Create example of traefik on host network for obtaining ingress source IP's

[BretFisher]

The ingress routing mesh loses the client source IP, and some web apps need this. This enhancement would add an example/option for placing traefik on --network host so it can capture those source IP's and send them as host headers to backend services on an overlay network.

With traefik this makes the whole thing a bit more complex because we now must have a traefik replica on every node (global mode) which means we can't cheat with the TOML config file and cert files like we can with a single-replica traefik service. We'll need a config backend for traefik to make this work.

[BretFisher]

Fixed in the big proxy update! 🎉 97a27e1