Automatic merge from submit-queue (batch tested with PRs 54160, 54016). If you want to cherry-pick this change to another branch, please follow the instructions <a href="https://github.com/kubernetes/community/blob/master/contributors/devel/cherry-picks.md">here</a>.
Fix `kubeadm upgrade plan` for offline operation
**What this PR does / why we need it**:
This PR allows `kubeadm upgrade plan` to work in firewalled/offline/otherwise restricted environments by ignoring errors when trying to reach dl.k8s.io. Instead, we fall back to the current kubeadm version as the latest stable version. This is a reasonable as a user is [expected to install a recent version of kubeadm before upgrading](https://kubernetes.io/docs/tasks/administer-cluster/kubeadm-upgrade-1-8/#upgrading-your-control-plane).
**Which issue this PR fixes**: Fixeskubernetes/kubeadm#498
**Special notes for your reviewer**: Should preferably be cherrypicked to 1.8.
```release-note
Fix `kubeadm upgrade plan` for offline operation: ignore errors when trying to fetch latest versions from dl.k8s.io
```
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