Use @ prefix for variables in erb templates to fix deprecated syntax

Puppet variables used in erb templates should have the '@' prefix set, to separate them from local ruby variables. This change silences deprecation warnings on the puppetmaster, such as these : 
```
Variable access via 'content' is deprecated. Use '@content' instead. template[/etc/puppet/modules/portage/templates/makeconf.conf.erb]:1
    (at /etc/puppet/modules/portage/templates/makeconf.conf.erb:1:in `result')
Variable access via 'name' is deprecated. Use '@name' instead. template[/etc/puppet/modules/portage/templates/makeconf.conf.erb]:4
    (at /etc/puppet/modules/portage/templates/makeconf.conf.erb:4:in `result')

```