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?
Okay -- to be clear -- I'd do something like this.  If there is a pillar for `external_cert_name` then set a local variable (also `external_cert_name`?) to that value.  If there we are GCE, set a local variable to some sentinel value (`_use_gce_external_ip`?).  If we are vagrant, then set that local variable to `grains.fqdn_ip4`.

Then, if that local variable is defined, call `make-ca-cert.sh` with just that single bit of context.  We don't even need to use jinja there as you can pass it in as an argument to the script.  Inside the script detect the GCE sentinel value and only then call out to the metadata server.

All a suggestion but it would make the per-cloud configuration exist just in the sls file and make the script much cleaner.