MB-100: tune xperf benchmark config

Change-Id: Ic9210a3c6dda8762597eebc22f56ca5ce3202a5a
Reviewed-on: http://review.couchbase.org/24313
Reviewed-by: Pavel Paulau <pavel.paulau@gmail.com>
Tested-by: Pavel Paulau <pavel.paulau@gmail.com>