Automatic merge from submit-queue

kubeadm: must lower-case token portion used in DNS label.

**What this PR does / why we need it**: In Kubernetes, DNS labels must be lower-case. `kubeadm` doesn't care when creating certain objects through the API. This PR fixes that erroneous behavior.

**Which issue this PR fixes**: fixes https://github.com/kubernetes/kubeadm/issues/104

**Special notes for your reviewer**: /cc @luxas @mikedanese @dgoodwin