We currently expose `ourpkgs` to nixos modules via an `ourpkgs` argument using
`_module.args`. However, `_module.args` is not available in the final
configuraiton. Thus, to allow tests to access the value, we also expose
is via passthru.

... to be in sync with https://github.com/PrivateStorageio/PaymentServer/pull/98

Since !146, PaymentServer is proxied behind nginx, so there is no need to wait
for certificates or network interfaces before starting the server.

Write Python with Nix (?).

I hadn't understood: The PaymentServer turns 16 vouchers into
50 k ZKAPs.  So far, we don't have a metric for used up ZKAPs.

Also, since the dashboard is already a full browser page and
only handling payment, I rename the services dashboard to payment
dashboard.

Fixes privatestorageops#349.

... so it can be used in the nginx config instead of duplicating the number

As per @jcalderone and @florian videoconf on 2021-08-06.

- Remove custom handling of Let's Encrypt certs, use NixOs' ACME
  integration instead.

- Use NGINX for TLS termination.

Tryfix. Can't run this locally ATM.

System tests do currently not include testing our monitoringvpn wireguard
setup, so let's just add the required IP by an alias to the lo interface.

This should fail currently, since metrics are still public.
Also, I couldn't test this code locally, since I currently run VirtualBox VMs
on my dev box. I'll just see what CI says to this code.

We have to explicitely add the port number so the relabel
config can strip it out again.  Else, `instance' seems to
get its value from somewhere else - somewhere our relabel
config does not look at.