Each secret file to be deployed is now backed by one local file.
This simplifies 'setup-secrets' and the secret definitions.
Also, with the old format it was not possible to add new secrets
to secrets.nix in a simple way.
Old secrets are automatically converted to the new format when running
nix-shell.
Using the new option 'nix-bitcoin.secrets', secrets are now directly
defined by the services that use them.
Without this, starting the virtualbox guest service fails during machine activation.
This is due to an incompatible NixOS machine base image.
Fix this by using an updated version of nixops.
