Upgrade system test software and port system tests from Perl to Python

Closes #41

See merge request privatestorage/PrivateStorageio!151

Write Python with Nix (?).

Services Dashboard: Payments

Closes privatestorageops#290 and privatestorageops#349

See merge request privatestorage/PrivateStorageio!154

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.

Clean up nix-store on the machines with limited disk space

See merge request privatestorage/PrivateStorageio!149

Less swap on disk constrained AWS hosts

See merge request privatestorage/PrivateStorageio!150

... 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.

Update nixpkgs-ps to include PaymentServer 0.1.1.1

See merge request privatestorage/PrivateStorageio!147

... fixing the Prometheus metric names a second time.

Prometheus niceties

See merge request privatestorage/PrivateStorageio!145

NGINX does longest-prefix-match for selecting locations.

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.