selftests: openvswitch: add ct-nat test case with ipv4

Building on the previous work, add a very simplistic NAT case
using ipv4.  This just tests dnat transformation

Signed-off-by: Aaron Conole <aconole@redhat.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>