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/node/templates/kubelet.service.j2

@@ -10,7 +10,7 @@ Wants=docker.socket
 {% endif %}
 
 [Service]
-EnvironmentFile=/etc/kubernetes/kubelet.env
+EnvironmentFile={{kube_config_dir}}/kubelet.env
 ExecStart={{ bin_dir }}/kubelet \
 		$KUBE_LOGTOSTDERR \
 		$KUBE_LOG_LEVEL \