Just require hostName and domain instead of an extra publicIPv4 option.  Leave
behind a similar option, `publicAddress`, just in case it needs to be
overridden at some point, but stop supplying a value for it anywhere.

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

Also configure a new value on the staging grid

And test for (kind of) SSH availability

This gives us a good opportunity to mention the privacy policy, too.

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

It knows better than we do about its own dependencies.  Let it control them.

Also open the firewall!  Yea.  That is a pretty important thing.

This results in less repetition, less abstraction leakage, and provides more
power to the private-storage module (by exposing meaningful values instead of
Tahoe-LAFS config strings).