system-ovn.at: Add a OVN NAT test using OVN gateway.

system-ovn.at: Add a OVN NAT test using OVN gateway.

This unit test adds a basic OVN NAT test that tests north-south
DNAT, south-north SNAT and east-west DNAT and SNAT. It uses network
namespaces connected to br-int using veth pairs to act as logical
ports. This test does not cover multi-host scenarios, so there is
a gap. But userspace OVN tests do multi-host scenarios (without NAT
testing), so it should still be a decent coverage.

Signed-off-by: Gurucharan Shetty <guru@ovn.org>
Acked-by: Joe Stringer <joe@ovn.org>