Refactoring SSL setup

- Fixing `if ssl` -> `if $ssl` (not a variable comparison)
- Comparing `$ssl` to `undef`, not `yes`, so that `required` can be used
  as well
- Single quotes for `ssl_cert`, `ssl_key`, and `ssl_ca` are unnecessary per
  the [Dovecot Wiki](http://wiki2.dovecot.org/SSL/DovecotConfiguration)
- For any setting that is `undef`, the respective configuration is removed

This PR is clean as well.