balancedconsumer: bugfix retrieving zk partitions
balancedconsumer: bugfix retrieving zk partitions

This is borrowed from _get_participants(); it is a situation that
probably occurs mostly in the test suite rather than anywhere else.

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

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