Add support for remapping brokers based on node id

[mikekap]

Kafka allows brokers to advertise any IP, but they must have a consistent node id. It would be useful if kafka-proxy could key the IP replacement logic by node id as well as ip. For example, something like --bootstrap-server-mapping=NODE_ID:1,localhost:30001.

This would still require at least one --bootstrap-server-mapping that's not node id based, but it would provide a nice alternative in case the advertised hostnames aren't readily available and change over time.