Implement nested snat validation test scenario

It's disabled for ovn because it's not, yet, implemented for the driver.

See: https://review.opendev.org/c/openstack/neutron/+/926495

(The feature also requires special configuration for the driver that we
probably won't enable in the gate. This can be discussed though.)

Related-Bug: #2051935
Change-Id: Ie2e49a53857009446e22300e2fff292355cef058
3 files changed