ovn-northd: Force SNAT for multiple gateway routers.

ovn-northd: Force SNAT for multiple gateway routers.

When multiple gateway routers exist, a packet can
enter any gateway router. Once the packet reaches its
destination, its reverse direction should be via the
same gateway router.  This is achieved by doing a SNAT
of the packet in the inward direction (towards logical space)
with a IP address of the gateway router such that packet travels back
to the same gateway router.

To do the above, we introduce two new options in the logical router.

options:dnat_force_snat_ip=$IP will force SNAT any packet to $IP if
it has been previously DNATted.

options:lb_force_snat_ip=$IP will force SNAT any packet to $IP if
it has been previously load-balanced.

Signed-off-by: Gurucharan Shetty <guru@ovn.org>
Acked-by: Mickey Spiegel <mickeys.dev@gmail.com>