kafka_instance: don't delete internal topics
kafka_instance: don't delete internal topics

... because the management script shipping with kafka 0.9 forbids this.
With this change, I can now complete a full test suite run against a
kafka 0.9.0.0 cluster.  I had expected to see issues caused by the
"__consumer_offsets" topic getting left intact, but apparently that's
fine.

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

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