specify image-repository fo kubeadm

--image-repository=registry.cn-hangzhou.aliyuncs.com/google_containers

add a trailling slash in the url like

registry.cn-hangzhou.aliyuncs.com/google_containers

will cause failure

failed to pull image "registry.cn-hangzhou.aliyuncs.com/google_containers//kube-apiserver:v1.19.2": output: invalid reference format