added support for external zookeeper

* cleanup in start-kafka script
* removed obsolete script
don't force zookeeper to listen on host's port 2181 since we're connecting directly to the zookeeper container
allow kafka container to connect to multiple zookeeper servers via docker links
allow our generated attributes to be overridden by external environment variables
automatically delete the kafka-shell container when exiting
allow broker-list to show all running kafka brokers
allow the logs directory to be mounted from an external volume