Add Nginx front-end

[tgross]

Jenkins requires SSL to be operated securely. You should only run Jenkins behind a reverse proxy that supports SSL (ex. Nginx).

Once we have a blueprint for Let's Encrypt support in Nginx (ref autopilotpattern/nginx#25) we should give this Jenkins blueprint an Nginx front-end.

[tgross]

Like with #7, the initial implementation of doing this will be in https://github.com/joyent/product-automation (private) which we'll be using for CI/CD on Joyent.com. I'll work thru backporting that into this repo once that work is done and stable and tested.