Adds kubeadm subcommands to create, list, and delete bootstrap tokens.
Tokens can be created with a TTL duration, or 0 for tokens that will not
expire. The create command can also be used to specify your own token
(for use when bootstrapping masters and nodes in parallel), or update an
existing token's secret or ttl.
Marked "ex" for experimental for now as the boostrap controllers are not
yet hooked up in core.
