Add "disconnect" action to the validation framework

[ipspace]

When we're testing the effects of an interface failure in the validation framework, we shut down the interface on the probe node, potentially resulting in other interfaces being shut down if you use Arista EOS as probes.

Samuel K Lam made a great suggestion in a LinkedIn comment: we should shut down the vEth/tap interface instead.

That would require a new action in the validation framework, but also significantly simplify some of the validation tests as we would no longer rely on "ifupdown" custom configs to get the job done.