nixos/tests: Add common modules for letsencrypt

These modules implement a way to test ACME based on a test instance of
Letsencrypt's Boulder service. The service implementation is in
letsencrypt.nix and the second module (resolver.nix) is a support-module
for the former, but can also be used for tests not involving ACME.

The second module provides a DNS server which hosts a root zone
containing all the zones and /etc/hosts entries (except loopback) in the
entire test network, so this can be very useful for other modules that
need DNS resolution.

Originally, I wrote these modules for the Headcounter deployment, but
I've refactored them a bit to be generally useful to NixOS users. The
original implementation can be found here:

https://github.com/headcounter/deployment/tree/89e7feafb/modules/testing



Quoting parts from the commit message of the initial implementation of
the Letsencrypt module in headcounter/deployment@95dfb31110397567534f2:

    This module is going to be used for tests where we need to
    impersonate an ACME service such as the one from Letsencrypt within
    VM tests, which is the reason why this module is a bit ugly (I only
    care if it's working not if it's beautiful).

    While the module isn't used anywhere, it will serve as a pluggable
    module for testing whether ACME works properly to fetch certificates
    and also as a replacement for our snakeoil certificate generator.

Also quoting parts of the commit where I have refactored the same module
in headcounter/deployment@85fa481b3431bbc450e8008fd25adc28ef0c6036:

    Now we have a fully pluggable module which automatically discovers
    in which network it's used via the nodes attribute.

    The test environment of Boulder used "dns-test-srv", which is a fake
    DNS server that's resolving almost everything to 127.0.0.1. On our
    setup this is not useful, so instead we're now running a local BIND
    name server which has a fake root zone and uses the mentioned node
    attribute to automatically discover other zones in the network of
    machines and generate delegations from the root zone to the
    respective zones with the primaryIPAddress of the node.

    ...

    We want to use real letsencrypt.org FQDNs here, so we can't get away
    with the snakeoil test certificates from the upstream project but
    now roll our own.

    This not only has the benefit that we can easily pass the snakeoil
    certificate to other nodes, but we can (and do) also use it for an
    nginx proxy that's now serving HTTPS for the Boulder web front end.

The Headcounter deployment tests are simulating a production scenario
with real IPs and nameservers so it won't need to rely on
networking.extraHost. However in this implementation we don't
necessarily want to do that, so I've added auto-discovery of
networking.extraHosts in the resolver module.

Another change here is that the letsencrypt module now falls back to
using a local resolver, the Headcounter implementation on the other hand
always required to add an extra test node which serves as a resolver.

I could have squashed both modules into the final ACME test, but that
would make it not very reusable, so that's the main reason why I put
these modules in tests/common.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>