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/network_plugin/flannel/templates/flannel-pod.yml

@@ -3,7 +3,7 @@
   apiVersion: "v1"
   metadata:
     name: "flannel"
-    namespace: "kube-system"
+    namespace: "{{system_namespace}}"
     labels:
       app: "flannel"
       version: "v0.1"