Add automatic SSL handling

We would like for API Umbrella to optionally handle all of the SSL setup for any domains it's handling. This includes automatically registering new SSL certificates and renewing SSL certificates as necessary.

With the advent of Let's Encrypt (free and automated SSL registration/renewals) and OpenResty's ssl_certificate_by_lua (dynamic SSL certificates), this is now possible and makes SSL setup painless (particularly in multi-tenant environments where API Umbrella might be serving multiple domains).

We've already implemented the bulk of this functionality in the separate lua-resty-auto-ssl project. We've been piloting this in our api.data.gov installation of API Umbrella and it's been working well. So the remaining tasks here are mainly around integrating this functionality directly into API Umbrella.

Things to do:

• Open source the MongoDB storage adapter we're using on api.data.gov (current closed source location).
  • As part of this, it would be good to abstract our encrypt-at-rest feature in the MongoDB adapter into a higher-level lua-rest-auto-ssl feature. This would allow for a standardized and easy way to have any lua-resty-auto-ssl storage adapter encrypt its data.
• Integrate an allow_domain function for lua-resty-auto-ssl that uses the list of API Backends and Website Backends from API Umbrella to determine what domains should be allowed. This is the one piece of functionality we're currently missing from our api.data.gov pilot implementation, but this is a key piece to making this even easier and fully automatic (so as soon as an API backend is added, then we would allow for its cert to be registered).

Start by reading the lua-resty-auto-ssl project and the current adapter at files/default/api-umbrella-mongodb.lua. Map how API Backends and Website Backends are represented, then define completion as an open MongoDB storage adapter with the proposed encryption abstraction and an allow_domain integration for permitted domains.