allow to configure how ns_process_registry terminates children

Change-Id: I37bf64eda9226585fbd41a5cafe4bce2d404e3ad
Reviewed-on: http://review.couchbase.org/38584
Tested-by: Artem Stemkovski <artem@couchbase.com>
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>