ovn: Add software l2 gateway.

ovn: Add software l2 gateway.

This patch implements one approach to using ovn-controller to implement
a software l2 gateway between logical and physical networks.

A new logical port type called "l2gateway" is introduced here.  It is very
close to how localnet ports work, with the following exception:

- A localnet port makes OVN use the physical network as the
  transport between hypervisors instead of tunnels. An l2gateway port still
  uses tunnels between all hypervisors, and packets only go to/from the
  specified physical network as needed via the chassis the l2gateway port
  is bound to.

- An l2gateway port also gets bound to a chassis while a localnet port does
  not.  This binding is not done by ovn-controller.  It is left as an
  administrative function.  In the case of OpenStack, the Neutron plugin
  will do this.

Signed-off-by: Russell Bryant <russell@ovn.org>
Acked-by: Ryan Moats <rmoats@us.ibm.com>
Acked-by: Ben Pfaff <blp@ovn.org>
Acked-by: Justin Pettit <jpettit@ovn.org>