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>

hosts/openstack-template/default.nix

@@ -0,0 +1,7 @@
+{ ... }:
+{
+  imports = [
+    ./configuration.nix
+    ../../system/packages.nix
+  ];
+}