simplify child_erlang:open_port_args

Change-Id: Ic1461f74a40d15342b9340dafd5ae25c0e8fdcfe
Reviewed-on: http://review.couchbase.org/40808
Tested-by: Artem Stemkovski <artem@couchbase.com>
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>