ovn: Send GARP for the router ports with reside-on-redirect-chassis options set

ovn: Send GARP for the router ports with reside-on-redirect-chassis options set

With the commit [1], the routing for the provider logical switches
connected to a router is centralized on the master gateway chassis
(if the option - reside-on-redirect-chassis) is set. When the
failover happens and a standby gateway chassis becomes master,
it should send GARPs for the router port macs. Without this, the
physical switch doesn't learn the new location of the router port macs
immediately and this could result in traffic disruption.

This patch addresses this issue so that the ovn-controller which claims the
distributed gatweway router port sends out the GARPs.

ovn-controller sends the GARPs if the Port_Binding.nat_addresses column
is set. This patch makes use of this column, instead of adding a new column
even though the name - nat_addresses seems a bit misnomer. The documentation is
updated to highlight the usage of this column.

This patch doesn't handle sending the GARPs for the gateway router port IPs.
This will be handled in a separate patch.

[1] - 85706c34d53d ("ovn: Avoid tunneling for VLAN packets redirected to a gateway chassis")

Acked-by: Dumitru Ceara <dceara@redhat.com>
Signed-off-by: Numan Siddique <nusiddiq@redhat.com>
Signed-off-by: Ben Pfaff <blp@ovn.org>