This removes the code that might delete a Tahoe-LAFS nodedir and replaces it
with code that will try to save an unexpected copy of it out of the way
instead.

And test for (kind of) SSH availability

It fails

nixpkgs Python tools changed to no longer set PYTHONHOME for us
automatically.

The layer above will have to make sure that it is.

also, make it work.

also if get-passes fails dump the tahoe logs

it needs the plugin too

at least, maybe.  we don't actually get any zkaps from this as of now.