cluster: update warning about empty broker metadata
cluster: update warning about empty broker metadata

Auto-creation on a fresh cluster has been possible as of 6768598.

Signed-off-by: Yung-Chin Oei <e4c1ab9da74060df152545062ee274845648ed6c@yungchin.nl>

wikimedia/operations-debs-python-pykafka,benauthor/pykafka,thedrow/samsa,yungchin/pykafka,benauthor/pykafka,thedrow/samsa,wikimedia/operations-debs-python-pykafka,thedrow/samsa,yungchin/pykafka,wikimedia/operations-debs-python-pykafka,benauthor/pykafka