Configurable cluster domain

[stephank]

This adds support for cluster domains other than cluster.local.

This feels hacky, but I didn't really find a clear path to passing some variable to ReverseProxyPlugin without introducing a global.

We can probably default this to the empty string. It should be possible to resolve e.g. kubernetes.default.svc inside a pod, because kubelet sets options ndots:5 in /etc/resolv.conf. Making it default empty means it should work for any domain without configuration.

In my case, though, I need it to be an FQDN, because I'm running caddy-ingress outside the cluster, and my stub resolver is systemd-resolved, which has explicitely rejected support for ndots. (They deem it a security concern.) So I need an option like this.

Keeping the cluster.local default seemed like the path of least resistance, for now. 🙃

[stephank]

#310 would also make this obsolete, but this may still be a more palatable short-term solution.