Lookup Kafka brokers from Zookeeper

Create a new ```Herman::Providers::JavaProducer``` object with the zookeeper info instead of broker ids.

This removes the functionality of instantiating the JavaProducer object with broker ids.
`broker_list` is a short enough call, any reason why we should bother saving the response into a temporary variable? IMO it'll fit just fine in the `#create_properties` call