Revert "Set exact user for Kubelet services" (#8872)

This reverts commit e3756786.

The workaround of explicitly specifying root for the kubelet unit was
for pulling images from private registry. Kubernetes now have a
dedicated mechanism with imagePullSecret.

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

@@ -9,7 +9,6 @@ Wants={{ container_manager }}.service
 {% endif %}
 
 [Service]
-User=root
 EnvironmentFile=-{{ kube_config_dir }}/kubelet.env
 ExecStart={{ bin_dir }}/kubelet \
 		$KUBE_LOGTOSTDERR \