Add cluster_partition_handling flag

This will allow a user to specify if they want to handle
network paritioning in the cluster. Default is nil, which
should perform exactly as the recipe does today.

For more info on partition handling, please see:
http://www.rabbitmq.com/partitions.html

Change-Id: Ib1dddefc67be3cc09d64dab28308de6c30161a6c