In some places of which network environment was limited, kubeadm
can't pull images from k8s.gcr.io. This patch add a variable
`KURYR_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: Id966747d8c09ea14bb37880f8fb37447591564b5
8655bb49c8