Add an option to validate the CA Cert of the master using a hash in `kubeadm join`

Proposal: https://docs.google.com/document/d/1SP4P7LJWSA8vUXj27UvKdVEdhpo5Fp0QHNo4TXvLQbw/edit?ts=5971498a
PR: https://github.com/kubernetes/kubernetes/pull/49520#issuecomment-319145433

@mattmoyer will later convert the proposal to markdown and check into git.