Check firewall openings when validating cluster settings

[joostdecock]

It is almost certain that people are going to run into firewall trouble when setting up a Morio cluster.
We already check that we can reach other nodes over HTTPS, but we should extend that to other ports, for Kafka and so on.

To do that, we could configure the proxy service to listen to these ports in ephemeral mode.
That way, we can just run an HTTP check to see if connection can be established using the ports required for inter-node cluster communication.