Use '@var' in templates to access variables

The following warning were emitted while compiling the catalog

```
Warning: Variable access via 'name' is deprecated. Use '@name' instead. template[/tmp/vagrant-puppet/modules-0/python/templates/gunicorn.deb.init.erb]:4
   (at /tmp/vagrant-puppet/modules-0/python/templates/gunicorn.deb.init.erb:4:in `result')
```

To be up-to-date with current puppet this patch changes the variables to their instance-variable equivalent.