Changes relative to upstream:
* Add explanatory comments
* Do not use stderr output of preKubeadmCommands indicate an error with
  bootstrapping

Changes relative to our fork:
* Do not enable IPv6
* Do not remove cloud-init logs and seed
* Do not disable VMware customization
* Do not disable network configuration
* Do not truncate cloud-init-output.log
* Do not report status of HTTP proxy configuration
* Do not configure cloud-init to remove SSH keys on first boot
* Remove commands that are already executed as a result of being defined in `preKubeadmCommands`

* Use shell script to clean cloud-init cache and reboot.
* Fix error handling of bootstrap script. Do not interpret stderr output
  as an indicator of failure. Do not rely on trap and errexit, because
  it does not work for command lists.
* Include last lines of output for error context.
* Ensure we have an IPv4 address for localhost.
* Remove unnecessary cloud-init configuration to preserve SSH host keys.