cluster: fix bug with internal topic lookups
cluster: fix bug with internal topic lookups

As demonstrated in the parent commit, this would previously allow
infinite recursion.  Fixes #277.

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

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