upgrading, and deleting clusters.

For the 1.8 release, SIG Cluster Lifecycle continued to focus on expanding the
capabilities of kubeadm, which is both a user-facing tool to manage clusters
and a building block for higher-level provisioning systems. Starting
with the 1.8 release, kubeadm supports a new upgrade command and includes alpha
support for self hosting the cluster control plane.

- github.com/mholt/certmagic: [6a42ef9](https://github.com/mholt/certmagic/tree/6a42ef9)
- github.com/naoina/go-stringutil: [v0.1.0](https://github.com/naoina/go-stringutil/tree/v0.1.0)
- github.com/naoina/toml: [v0.1.1](https://github.com/naoina/toml/tree/v0.1.1)
- github.com/robfig/cron: [v1.1.0](https://github.com/robfig/cron/tree/v1.1.0)