CBQE-1378 Adjust number of items for cbworkloadgen test

Change-Id: I561fb7aff9bda6ae98455548ef9b3ce959c863cc
Reviewed-on: http://review.couchbase.org/27756
Reviewed-by: Chisheng Hong <chisheng@couchbase.com>
Tested-by: Chisheng Hong <chisheng@couchbase.com>