This is a follow-up to !166 (c9ddc8ad), where I missed one occurence.

This change reflects my intention of having an alert fire when a node
is not scrapable for five minutes or longer.  I hadn't understood the
default avg() function well enough before, yielding an alert that would
fire too soon (i.e. any time a node was down, even only one minute for a
reboot).

Fixes privatestorageops#358.

- The serivice can be reached by other machines.
- The metrics cannot be reached by other machines.

This changes the Throughput graph to be relative, changes its unit
to Bit/s, and fixes the labels of the Network pkt/s graph.

Fixes privatestorageops#24.

Fixes privatestorageops#355.

[Copy pasta](https://whetstone.privatestorage.io/privatestorage/PrivateStorageio/-/blob/ca44204076983ce02fd3e0dacd940514e0821e23/nixos/modules/spending.nix#L130) - Thanks @tomprince!

Epiphany: Systemd reads more than a single environment file. This
lets me improve the file naming much.

Unfortunately still prints a bold warning on deployment.

This will store the supposedly secret URL in the Nix Store and
print a honking big warning about it.

Only dashboards in this dir

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

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.

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