ovn-northd: Can't use ct() for router ports.

ovn-northd: Can't use ct() for router ports.

This patch ensures that we do not attempt to use connection tracking for
logical ports with type=router.  This does not work as the traffic
through a logical router port is not symmetric since logical routers are
distributed.  The result was that traffic between logical ports on
different hypervisors that went through a logical router would fail if
ACLs were in use.

GitHub-PR: #92
Reported-at: https://bugs.launchpad.net/networking-ovn/+bug/1522022
Signed-off-by: l0310 <liw@dtdream.com>
[russell@ovn.org updated commit message, style tweaks]
Signed-off-by: Russell Bryant <russell@ovn.org>