mjolnir: use cloud-init for network configuration and move to new net.

Switch to cloud-init to align with the Ansible infra.
Also move to new network and hostname.

config/hosts/mjolnir/networking.nix

@@ -1,18 +1,24 @@
 { ... }:
 
 {
-  networking = {
-    interfaces.net0 = {
-      ipv4.addresses = [
-        {
-          address = "172.31.17.161";
-          prefixLength = 25;
-        }
+  networking.useDHCP = false;
+
+  services.cloud-init = {
+    enable = true;
+    network.enable = true;
+
+    # Version without ssh of:
+    # https://github.com/NixOS/nixpkgs/blob/3c9db02515ef1d9b6b709fc60ba9a540957f661c/nixos/modules/services/system/cloud-init.nix#L145
+    # So we don't get ssh hostkey regenerations.
+    settings.cloud_config_modules = [
+        "disk_setup"
+        "mounts"
+        "ssh-import-id"
+        "set-passwords"
+        "timezone"
+        "disable-ec2-metadata"
+        "runcmd"
       ];
-    };
-    defaultGateway = "172.31.17.129";
-    nameservers = [ "212.12.50.158" "192.76.134.90" ];
-    search = [ "hamburg.ccc.de" ];
   };
 
   systemd.network.links."10-net0" = {