ovn: Introduce l3 gateway router.

ovn: Introduce l3 gateway router.

Currently OVN has distributed switches and routers. When a packet
exits a container or a VM, the entire lifecycle of the packet
through multiple switches and routers are calculated in source
chassis itself. When the destination endpoint resides on a different
chassis, the packet is sent to the other chassis and it only goes
through the egress pipeline of that chassis once and eventually to
the real destination.

When the packet returns back, the same thing happens. The return
packet leaves the VM/container on the chassis where it resides.
The packet goes through all the switches and routers in the logical
pipleline on that chassis and then sent to the eventual destination
over the tunnel.

The above makes the logical pipeline very flexible and easy. But,
creates a problem for cases where you need to add stateful services
(via conntrack) on switches and routers.

For l3 gateways, we plan to leverage DNAT and SNAT functionality
and we want to apply DNAT and SNAT rules on a router. So we ideally need
the packet to go through that router in both directions in the same
chassis. To achieve this, this commit introduces a new gateway router which is
static and can be connected to your distributed router via a switch.

To make minimal changes in OVN's logical pipeline, this commit
tries to make the switch port connected to a l3 gateway router look like
a container/VM endpoint for every other chassis except the chassis
on which the l3 gateway router resides. On the chassis where the
gateway router resides, the connection looks just like a patch port.

This is achieved by the doing the following:
Introduces a new type of port_binding record called 'gateway'.
On the chassis where the gateway router resides, this port behaves just
like the port of type 'patch'. The ovn-controller on that chassis
populates the "chassis" column for this record as an indication for
other ovn-controllers of its physical location. Other ovn-controllers
treat this port as they would treat a VM/Container port on a different
chassis.

Signed-off-by: Gurucharan Shetty <guru@ovn.org>
Acked-by: Ben Pfaff <blp@ovn.org>