MB-8234: tune write ratio in DGM xperf config

Change-Id: I86ab2569a0bf1e0387720d7f95d38afdb1fc6b0a
Reviewed-on: http://review.couchbase.org/26463
Reviewed-by: Pavel Paulau <pavel.paulau@gmail.com>
Tested-by: Pavel Paulau <pavel.paulau@gmail.com>