[META] SSL refactor

SSL in Aegir is currently confusing to setup, requires command-line access to deploy keys, and is fairly fragile. This is reflected in lots of open issues relating to SSL in our various queues: https://www.drupal.org/project/issues?text=ssl&projects=provision%2C+hos...

We need to fix this; ideally for Aegir 3. I propose consolidating some of these issues and taking the following approach:

1. Move to SNI exclusively
2. Drop IP address handling (almost) entirely
3. Provide front-end forms for uploading or pasting certificate files
4. Validate these certificates before attempting to deploy them
5. Provide useful, up-to-date docs for SSL management in Aegir

(1) and (2) should greatly simplify SSL deployments, and should probably be the priority. According to this comment, there may not be much left to actually do here.

(3) should simply provide something like:

(4) would might require a bit more work, as I haven't yet found a native PHP method to validate certs. This probably just needs a bit more digging. On the backend, we could just make a system call to `openssl`, but we probably want to avoid that and use some of PHP's openssl_*() functions instead.

(5) would also be greatly simplified by (1) and (2).