fix loading of cluster_domain

Change-Id: I287782c0144ba3eec8fbdb1fd24d7bda9844b439
diff --git a/metadata/service/pool/single.yml b/metadata/service/pool/single.yml
index 4e7a736..05f1223 100644
--- a/metadata/service/pool/single.yml
+++ b/metadata/service/pool/single.yml
@@ -21,7 +21,7 @@
       address: 0.0.0.0
       cluster_dns: 10.254.0.10
       allow_privileged: True
-      cluster_domain: ${param:kubernetes_cluster_domain}
+      cluster_domain: ${_param:kubernetes_cluster_domain}
       kubelet:
         config: /etc/kubernetes/manifests
         allow_privileged: True