MB-9468: implement ability to specify extra args for child ports

I.e. so that we folks can tweak connections count, threads count etc
with ease.

Change-Id: I0dd33d94d91e3d999cac7c06d9a48177b0405d80
Reviewed-on: http://review.couchbase.org/30106
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>