Revert unnecessary `$default_ssl_vhost` validation change.

The `validate_bool()` call was moved in
112437730f70e18daf3aed9cedffd0c46348204d making it FreeBSD centric, but
it should be validated outside of any other condition.
Merge pull request #453 from dbeckham/fix_broken_validate_ssl_vhost

Revert unnecessary `$default_ssl_vhost` validation change.