ovn-northd: Optimize acl of localnet-port.

ovn-northd: Optimize acl of localnet-port.

Localnet port is not an endpoint, and have no security requirements
to use localnet port at present. So, for performance consideration, we
could do not use ct for localnet port.

The more specific discussion can be found from
https://mail.openvswitch.org/pipermail/ovs-dev/2017-July/335048.html

Signed-off-by: wangqianyu <wang.qianyu@zte.com.cn>
Acked-by: Han Zhou <zhouhan@gmail.com>
Signed-off-by: Russell Bryant <russell@ovn.org>