default behavior does not change, it uses k8s.gcr.io by default
added two vars: KUBE_DOCKER_REGISTRY, KUBE_BASE_IMAGE_REGISTRY.
KUBE_BASE_IMAGE_REGISTRY is for base image registry of server binaries
KUBE_DOCKER_REGISTRY is for released images registry
user can interact with them by:
`KUBE_DOCKER_REGISTRY=### KUBE_BASE_IMAGE_REGISTRY=### make quick-release`
Signed-off-by: Hui Luo <luoh@vmware.com>
97087c66f7