openstack-template: add minimal NixOS image for OpenStack

Adds a new host configuration for building qcow2 images targeting
OpenStack (NREC). Uses a nixos user with SSH key and sudo instead
of root login, firewall enabled, and no internal services.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

flake.nix

@@ -218,6 +218,15 @@
             ./hosts/pn02
           ];
         };
+        openstack-template = nixpkgs.lib.nixosSystem {
+          inherit system;
+          specialArgs = {
+            inherit inputs self;
+          };
+          modules = commonModules ++ [
+            ./hosts/openstack-template
+          ];
+        };
       };
       packages = forAllSystems (
         { pkgs }: