added ability to customize specific disk sink options

Change-Id: Ia58fcdf3c4100c740bfdc9e09d8a7eb43a327f53
Reviewed-on: http://review.couchbase.org/38449
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Artem Stemkovski <artem@couchbase.com>