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 think no value (i.e.: fail fast) would be a better default than localhost. The 9092 port is probably okay to include as a default since that's typically what Kafka runs on.