Add a script for generating CA signed certs, and a client cert.

@filbranden @jbeda Take a look.  @derekwaynecarr is there an easy way to get the IP address on a vagrant cluster?
I'd prefer that we don't hard code vagrant here but instead key off of the existence of something like `external_cert_name`.  That way if via some means people are able to give the cluster a name, we'll create a cert for it.