1.14: "upgrade plan" does not work correctly

[neolit123]

while testing in #1469 it was discovered that upgrade plan does not work with a kubeadm 1.14 binary over a existing 1.13.x cluster.

steps to reproduce:

• create a 1.13.5 single node cluster
• download a 1.14 kubeadm binary
• run sudo kubeadm upgrade plan

result is:

[preflight] Running pre-flight checks.
[upgrade] Making sure the cluster is healthy:
[upgrade/config] Making sure the configuration is correct:
[upgrade/config] Reading configuration from the cluster...
[upgrade/config] FYI: You can look at this config file with 'kubectl -n kube-system get cm kubeadm-config -oyaml'
[upgrade] Fetching available versions to upgrade to
[upgrade/versions] Cluster version: v1.13.5
[upgrade/versions] kubeadm version: v1.14.0

Awesome, you're up-to-date! Enjoy!

expected result:

• show what can be updated in the 1.13.5 cluster as in:
  https://kubernetes.io/docs/tasks/administer-cluster/kubeadm/kubeadm-upgrade-1-14/#upgrade-the-first-control-plane-node

WORKAROUND:
pass an explicit version e.g sudo kubeadm upgrade plan 1.14.0

/assign
@kubernetes/sig-cluster-lifecycle
potential candidate for cherry pick in release-1.14.

[neolit123]

fix will be available in 1.14.1 and 1.15.0.