CBES-8: support for es-node rebalance in/out

Change-Id: I4ced1e7c25ac5c00867801bfe002b860b1d11f4a
Reviewed-on: http://review.couchbase.org/23854
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Tommie McAfee <tommie@couchbase.com>