Kubeadm no longer supports kube-dns and CoreDNS is the only
supported DNS server. Remove ClusterConfiguration.DNS.Type
from v1beta3 that is used to set the DNS server type.
During upgrade the coredns migration library seems to require
that the input version doesn't have the "v" prefix".
Fixes a bug where the user cannot run commands such as
"kubeadm upgrade plan" if they have `v1.8.0` installed.
Assuming this is caused by the fact that previously the image didn't
have a "v" prefix.
