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 have a slight preference towards producing this pki/ directory in a temp directory or elsewhere, in any way outside of the checkout itself. Main concern again is the fact that it leaves the checkout dirty (check `git status` output.) One other possible solution is sending a commit upstream to include a `easyrsa3/.gitignore` file with `/pki` in it which should make it fine for the scripts to just create one there.