MB-100: define number of clients for load phase

Change-Id: I1377f0da8c7253c2ec22e38a53231891af9e187f
Reviewed-on: http://review.couchbase.org/23817
Reviewed-by: Pavel Paulau <pavel.paulau@gmail.com>
Tested-by: Pavel Paulau <pavel.paulau@gmail.com>