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



Do you think we should error out if there is a problem in parsing this entire thing. What if it's not 2, then we just skip and keep moving on. Do you think we may want to stop and error or at least tell something went wrong.