Fix deprecated variable access warning in vhost header template

Fixes the warning

```
Warning: Variable access via 'vhost_cfg_prepend' is deprecated. Use '@vhost_cfg_prepend' instead. 
template[/etc/puppet/modules/nginx/templates/vhost/vhost_header.erb]:17
   (at /etc/puppet/modules/nginx/templates/vhost/vhost_header.erb:17:in `result')