MB-7478: Remote reference to vbucket_mapping_rebalance

It's now cloned under ns_server/rebalancer. Rebase.

Change-Id: Id682b3864004312f75b18024313c92a0af081f61
Reviewed-on: http://review.couchbase.org/23680
Reviewed-by: Sriram Melkote <siri@couchbase.com>
Tested-by: Sriram Melkote <siri@couchbase.com>