Add support for flex volumes plugins.

roles/kubernetes/node/templates/kubelet.host.service.j2

@@ -6,6 +6,9 @@ Wants=docker.socket
 
 [Service]
 EnvironmentFile=-{{kube_config_dir}}/kubelet.env
+{% if kubelet_flexvolumes_plugins_dir is defined %}
+ExecStartPre=-/bin/mkdir -p {{ kubelet_flexvolumes_plugins_dir }}
+{% endif %}
 ExecStart={{ bin_dir }}/kubelet \
 		$KUBE_LOGTOSTDERR \
 		$KUBE_LOG_LEVEL \
@@ -17,6 +20,7 @@ ExecStart={{ bin_dir }}/kubelet \
 		$KUBELET_ARGS \
 		$DOCKER_SOCKET \
 		$KUBELET_NETWORK_PLUGIN \
+		$KUBELET_VOLUME_PLUGIN \
 		$KUBELET_CLOUDPROVIDER
 Restart=always
 RestartSec=10s