Use '@var' in templates to access variables

The following warning were emitted while compiling the catalog
```
Warning: Variable access via 'nginx_user' is deprecated. Use '@nginx_user' instead. template[/tmp/vagrant-puppet/modules-0/nginx/templates/nginx.conf.erb]:1
   (at /tmp/vagrant-puppet/modules-0/nginx/templates/nginx.conf.erb:1:in `result')
```
To be up-to-date with current puppet this patch changes the variables to their instance-variable equivalent.