This change adds the `--discovery-token-ca-cert-hash` and `--discovery-token-unsafe-skip-ca-verification` flags for `kubeadm join` and corresponding fields on the kubeadm NodeConfiguration struct. These flags configure enhanced TLS validation for token-based discovery.
The enhanced TLS validation works by pinning the public key hashes of the cluster CA. This is done by connecting to the `cluster-info` endpoint initially using an unvalidated/unsafe TLS connection. After the cluster info has been loaded, parsed, and validated with the existing symmetric signature/MAC scheme, the root CA is validated against the pinned public key set. A second request is made using validated/safe TLS using the newly-known CA and the result is validated to make sure the same `cluster-info` was returned from both requests.
This validation prevents a class of attacks where a leaked bootstrap token (such as from a compromised worker node) allows an attacker to impersonate the API server.
This change also update `kubeadm init` to print the correct `--discovery-token-ca-cert-hash` flag in the example `kubeadm join` command it prints at the end of initialization.
