Add createTopic() method
Improve and consolidate the Kafka + ZooKeeper setup

This commit notably includes:

- Consolidates Kafka/ZK cluster creation via EmbeddedKafkaZooKeeperCluster
- Add send() methods to KafkaProducerApp to specify target topic via
  parameters
- Better lifecycle management of Kafka consumer apps by adding startup()
  and shutdown() methods
Merge branch 'feature/kafka-zk-consolidation' into develop
Update CHANGELOG