Automatic merge from submit-queue

kubeadm: fix preflight checks

This PR fixes a couple issues cause by some bad rebases:
* When a pre-flight check returned errors, `kubeadm` would exit with error code `1` instead of `2` as the original pre-flight PR meant. This would also cause the output of `kubeadm` to include some stuff that was not supposed to be there.
* Duplicated `k8s.io/kubernetes/cmd/kubeadm/app/util` import.

I also took the freedom to do some output clean-up based on the input from the original pre-flight PR.

/cc @dmmcquay @dgoodwin @luxas