This adds a warning to `kubeadm init` and `kubeadm token create` if they are run without the `--token-ttl` / `--ttl` flags. In 1.7 and before, the tokens generated by these commands defaulted to an infinite TTL (no expiration) in 1.8, they will generate a token with a 24 hour TTL.
The actual default change is in https://github.com/kubernetes/kubernetes/pull/48783. This change is separate so we can cherry pick the warning into the release-1.7 branch.
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