Support gateways to private networks

[wallyqs]

Currently gateways require to use a public ip, should also support using $KUBERNETES_NODE_IP as the advertise address in case everything is in private networks.

[mtryfoss]

I think this already work as intended, after my PR?

It advertises the private IP of the Pod, which works perfectly fine in our environment. 2 x kubernetes + 2 x standalone installations in one big cluster using gateways.

However, I'm a bit uncertain of the best way to do initial discovery between two kubernetes clusters. I got it working using a loadbalancer with internal IP (Azure) with "externalTrafficPolicy: Local".

Using default setting here caused some sort of loop. Everything worked, but a constant error of some gateway trying to connect, but not answering to the PING after INFO message.