Introduce container_registry for image prefix

Change-Id: I80067f53aa2b63286b654fff782f0f7b65a2d050
diff --git a/zuul.d/project.yaml b/zuul.d/project.yaml
index b9c97e5..c28e328 100644
--- a/zuul.d/project.yaml
+++ b/zuul.d/project.yaml
@@ -157,3 +157,5 @@
             dependencies: *molecule_check_dependencies
         - atmosphere-molecule-aio-upgrade-ovn:
             dependencies: *molecule_check_dependencies
+    vars:
+      container_registry: harbor.atmosphere.dev/library