MB-100: use num_nodes_before instead of num_nodes

Change-Id: I0290690d543df3b463e6ce84e33fe54e7ab084fd
Reviewed-on: http://review.couchbase.org/25389
Reviewed-by: Pavel Paulau <pavel.paulau@gmail.com>
Tested-by: Pavel Paulau <pavel.paulau@gmail.com>