Address standalone kubelet config case

Also place in global vars and do not repeat the kube_*_config_dir
and kube_namespace vars for better code maintainability and UX.

Signed-off-by: Bogdan Dobrelya <bdobrelia@mirantis.com>

roles/kubernetes/master/vars/main.yml

@@ -3,4 +3,4 @@ namespace_kubesystem:
   apiVersion: v1
   kind: Namespace
   metadata:
-    name: kube-system
+    name: "{{system_namespace}}"