nixos/cloud-init: fix hostname and resolvconf configuration

- Fix hostname configuration on proxmox, which uses "hostname" in user-data
  instead of "local-hostname" in meta-data.
- Allow setting resolv.conf through cloud-init
- Add tests for new changes
- Add timeouts to make tests fail faster

nixos/tests/cloud-init-hostname.nix

@@ -0,0 +1,46 @@
+{ system ? builtins.currentSystem,
+  config ? {},
+  pkgs ? import ../.. { inherit system config; }
+}:
+
+with import ../lib/testing-python.nix { inherit system pkgs; };
+with pkgs.lib;
+
+let
+  # Hostname can also be set through "hostname" in user-data.
+  # This is how proxmox configures hostname through cloud-init.
+  metadataDrive = pkgs.stdenv.mkDerivation {
+    name = "metadata";
+    buildCommand = ''
+      mkdir -p $out/iso
+
+      cat << EOF > $out/iso/user-data
+      #cloud-config
+      hostname: testhostname
+      EOF
+
+      cat << EOF > $out/iso/meta-data
+      instance-id: iid-local02
+      EOF
+
+      ${pkgs.cdrkit}/bin/genisoimage -volid cidata -joliet -rock -o $out/metadata.iso $out/iso
+    '';
+  };
+
+in makeTest {
+  name = "cloud-init-hostname";
+  meta = with pkgs.lib.maintainers; {
+    maintainers = [ lewo illustris ];
+  };
+
+  nodes.machine2 = { ... }: {
+    virtualisation.qemu.options = [ "-cdrom" "${metadataDrive}/metadata.iso" ];
+    services.cloud-init.enable = true;
+    networking.hostName = "";
+  };
+
+  testScript = ''
+    unnamed.wait_for_unit("cloud-final.service")
+    assert "testhostname" in unnamed.succeed("hostname")
+  '';
+}