CBQE-1536 add method to return
the number of node(s) in cluster

Change-Id: Iba1e7e0a16f322c751b1673a6c9f654f2647cd3b
Reviewed-on: http://review.couchbase.org/29198
Tested-by: Thuan Nguyen <soccon@gmail.com>
Reviewed-by: Ketaki Gangal <ketakigangal@gmail.com>