Allow configuration of listeners

We want listeners in Kafka to be fully configurable. They should not be automatically added as they are now.
We will follow the Strimzi model (documented here: https://strimzi.io/docs/operators/latest/overview.html#configuration-points-broker_str) with the following exceptions:

• listener type: "route", "loadbalancer" and "ingress" types removed/not added.
• authentication type: only "tls" is supported for now, if nothing is specified no authentication is enabled
• we want to provide additional configuration within the authentication subsection when tls is chosen to specify the Secret where the necessary certificate chain (own cert + CA) and keys are stored (we want to be able to specify CA and certificate etc. in their own Secrets to be able to reuse the CA in other services easily)

I am unsure how well this fits into the product_config model - please talk to @maltesander before starting this task to fully flesh out the implementation details.

This ticket contains of at least two parts:

• Come up with a proposed CRD structure and put it up for review
• Implement everything APART from the TLS part, if someone choses "tls" it is fine to bail out/panic for now

Start by discussing the product_config model and implementation details with @maltesander, then review the linked Strimzi broker listener configuration. Propose the CRD structure for review before implementing configurable listeners, excluding TLS support for now. Done means the proposal is reviewed and non-TLS listener configuration is implemented.