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
  http://wiki2.dovecot.org/SSL/DovecotConfiguration
- For any setting that is 'undef', the respective configuration is removed
Merge pull request #6 from andschwa/ssl-fix

Refactoring SSL setup