nixos/security: add acme through dns challenge

few side refactors of this:
- no more `dns.domain`, it all must rely on `fqdn`, prevents
  inconsistencies.
- also added an specific host `tuxcord-acmetest` that uses the key zone
  for `nix.tuxcord.net` to test certificate pulling.

nixos/hosts/tuxcord-test/default.nix

@@ -1,4 +1,5 @@
 {
+  acme.enable = false;
   dns.enable = true;
   networking.fqdn = "tuxcord.test";
 }