MB-9468: implement ability to specify extra args for child ports
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 <340b8e09ca65cd3fc686427fcfed17e87eaf61e2@gmail.com>
Reviewed-by: Aliaksey Artamonau <3c875bcfb3adf2a65b2ae7686ca921e6c9433147@gmail.com>

nimishzynga/ns_server,nimishzynga/ns_server,ceejatec/ns_server,ceejatec/ns_server,daverigby/ns_server,t3rm1n4l/ns_server,mschoch/ns_server,mschoch/ns_server,membase/ns_server,daverigby/ns_server,membase/ns_server,t3rm1n4l/ns_server,daverigby/ns_server,membase/ns_server,nimishzynga/ns_server,daverigby/ns_server,nimishzynga/ns_server,nimishzynga/ns_server,mschoch/ns_server,t3rm1n4l/ns_server,ceejatec/ns_server,membase/ns_server,t3rm1n4l/ns_server,t3rm1n4l/ns_server,mschoch/ns_server,mschoch/ns_server,ceejatec/ns_server,ceejatec/ns_server,membase/ns_server,nimishzynga/ns_server,daverigby/ns_server,membase/ns_server,ceejatec/ns_server,mschoch/ns_server