Rack-Aware replica assignment option

Adding a rack-id to kafka config. This rack-id can be used during replica assignment by using the max-rack-replication argument in the admin scripts (create topic, etc.). By default the original replication assignment algorithm is used because max-rack-replication defaults to -1. max-rack-replication > -1 is not honored if you are doing manual replica assignment (preffered).

If this looks good I can add some test cases specific to the rack-aware assignment.