Make Kafka maintenance easier

This commit enables two settings which are useful when restarting a Kafka node
as part of a cluster. The first supports moving partitions for which a node
being restarting is the leader to another broker, and the second supports
automatically rebalancing partitions when the node rejoins the cluster. See
the documentation for more details:

https://kafka.apache.org/10/documentation.html#basic_ops_restarting

Change-Id: I0718fca08ea116db926b348e3e6e3bc2373db1fb

ansible/roles/kafka/templates/kafka.server.properties.j2

@@ -1,4 +1,6 @@
 listeners=PLAINTEXT://{{ api_interface_address }}:{{ kafka_port }}
+controlled.shutdown.enable=true
+auto.leader.rebalance.enable=true
 num.network.threads=3
 num.io.threads=8
 socket.send.buffer.bytes=102400