Merge most of the code from the `customize-*.nix` nodes into the base modules.

Part of #80 (closed).

This merges all the code from customize-issuer.nix and customize-storage.nix into the base modules, as well as the monitoring VPN secret keys from customize-monitoring.nix.

As part of working on privatestorageops#382/#84, I was looking at adding a new module parallel to the existing gridlib modules for the spending service. I wanted to avoid copy code that we wanted to refactor, so I went and did the refactoring the code first.