Remove config dependency on ZooKeeper (not ZooKeeper the service)

[ybyzek]

Instead of configuring a service to connect to ZK, connect to broker. (This GH issue is NOT to remove ZK from the Docker Compose files).

e.g. replace instances of:

SCHEMA_REGISTRY_KAFKASTORE_CONNECTION_URL: 'zookeeper:2181'

with

SCHEMA_REGISTRY_KAFKASTORE_BOOTSTRAP_SERVERS: 'broker:29092'

[ybyzek]

Along with the configuration change, there is one more unnecessary ZK dependency, and that is that Schema Registry container should rely just on the broker. If we could remove the depends_on ZooKeeper container (e.g. https://github.com/confluentinc/kafka-tutorials/blob/master/_includes/tutorials/cogrouping-streams/kstreams/code/docker-compose.yml#L43), that would be helpful as well.

[ybyzek]

@awalther28 Just as FYI, if #579 weren't already in-flight, this PR could have removed the ZK configuration from the Confluent Metrics Reporter as well (remove CONFLUENT_METRICS_REPORTER_ZOOKEEPER_CONNECT in favor of CONFLUENT_METRICS_REPORTER_BOOTSTRAP_SERVERS). However, since #579 is already in-flight, that may not be necessary in resolving this PR.