Legacy variable substitution (fix)

Ref #98 

Just wanted to update the library to use Ansible's latest variable substitution. Unfortunately, the [wordpress/templates/vhosts](https://github.com/genesis/wordpress/tree/master/provisioning/roles/wordpress/templates/vhosts) will trigger the legacy error due to Apache's own variable syntax `${APACHE_LOG_DIR}` which Ansible interprets as legacy code.