Hardening Kafka integration to Venice

Two key changes here:
- Allow Venice Consumer task to listen to multiple brokers and integrate with Kafka broker replication
- Provide kafka-broker-list to stores metadata, such that clients can produce to the correct topics



I use the brokerString to initialize the Kafka Producer, which looks for the format "host1:port1,host2:port2".

What happens is that the config input is a string like this, which I then parse and use to initialize the Kafka Consumers.