ovn-northd: Support connecting multiple routers to a switch.

ovn-northd: Support connecting multiple routers to a switch.

Currently we can connect routers via "peer"ing. This limits
the number of routers that can be connected with each other
directly to 2.

One of the design goals for L3 Gateway is to be able to
have multiple gateways (each with their own router)
connected to a distributed router via a switch.

With the above goal in mind, this commit gives the general
ability to connect multiple routers via a switch.

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