fix cn bug when generating tls assets using OpenSSL

[dixudx]

These hostnames should be explicitly shown to avoid potential misunderstandings.

[zbwright]

Hey - Thanks for your help on these. I'm closing this PR because we've completely rewritten and moved the Kubernetes documentation. Take a look, if you like: https://coreos.com/tectonic/docs/latest/tutorials/kubernetes/getting-started.html