The nixpkgs fork already contains all of the changes our overlays were
responsible for - but as direct nixpkgs changes instead.

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

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

also if get-passes fails dump the tahoe logs

it needs the plugin too

Put it into a language that I can more easily maintain

The issuer runs.  Helpers to poke the right API calls into the right place
forthcoming...

just need 35 more hosts and then defining the function will be a win ...