Fixes the following kubeadm warning:
W0321 01:33:46.409134 14953 checks.go:835] detected that the
sandbox image "registry.k8s.io/pause:3.6" of the container
runtime is inconsistent with that used by kubeadm.
It is recommended that using "registry.k8s.io/pause:3.9"
as the CRI sandbox image.
Change-Id: I8129a6e9ad3acdf314e2853851cd5274855e3209