Do not require host.name in server.properties

Kafka uses this attribute to determine which address to bind, and you need to be able to omit host.name to bind to all interfaces.

From the [Kafka 0.8 docs](https://kafka.apache.org/08/configuration.html): "Hostname of broker. If this is set, it will only bind to this address. If this is not set, it will bind to all interfaces, and publish one to ZK."