Allow configuration of listeners #141

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