follow new naming conventions for gcr's coredns image.
starting from 1.21 kubeadm assumes it to be `coredns/coredns`:
this causes the kubeadm deployment being unable to pull image, beacuse `v`
was also added in image tag, until the role `kubernetes-apps` ovverides
it with the old name, which is only compatible with <=1.7.

Backward comptability with kubeadm <=1.20 is mantained checking
kubernetes version and falling back to old names (`coredns:1.xx`) when
the version is less than 1.21