In some places of which network environment was limited, kubeadm
can't pull images from k8s.gcr.io. This patch add a variable
`KUBEADMIN_IMAGE_REPOSITORY` in order to the developer who located in
these places can set the kubeadm to pull container images from
repository that they can access.
Change-Id: I14aed50077ef0760635e575770fd2274cb759c53