Changes to make sure Kafka servers starts and registers with ZK

I would recommend testing (and saving the value of) `znode_exists?` for the conditional chain. We want the test and result to be atomic. Should the znode flap, we can have a non-obvious result. Given the following states:
# Znode not available in line 19 -> we advance to line 20
# Znode available in line 20 -> we advance to line 24
# Znode available in line 24 -> we advance to line 27
# Znode unavailable in line 27 -> we print "Broker #{@brokerid} existance : false" -- which makes no sense for the condition where we report success (e.g. it exists) & we advance to line 30
# In line 30 -> we print "Kafka is up and running." -- immediately after showing it was not; now we have a confused administrator.

I do not recommend more stringent Znode checking (e.g. re-entering the loop); should Kafka flap this way shame on it. But we should ensure our logic is atomic -- so please only check `znode_exists` once for each pass through the loop.