Our partiton rebalance mechanism broke on the upgrade from kafka-python 0.9.2
to 0.9.5. Rather than fiddling with the internals of the kafka consumer object
we're now reconstructing the consumer object after each rebalance and handing
it the specific partitions it needs to worry about.
Closes-bug: #1560178
Change-Id: I469ceb28538db1f36918f211eaea4fcfdaa17649