switch to new variables @ format

This is to remove warnings:
```
Warning: Config file /etc/puppet/hiera.yaml not found, using Hiera defaults
Warning: Variable access via 'log' is deprecated. Use '@log' instead. template[/tmp/vagrant-puppet-1/modules-0/logrotate/templates/conf.erb]:3
   (at /tmp/vagrant-puppet-1/modules-0/logrotate/templates/conf.erb:3:in `block in result')
Warning: Variable access via 'options' is deprecated. Use '@options' instead. template[/tmp/vagrant-puppet-1/modules-0/logrotate/templates/conf.erb]:6
   (at /tmp/vagrant-puppet-1/modules-0/logrotate/templates/conf.erb:6:in `block in result')
Warning: Variable access via 'prerotate' is deprecated. Use '@prerotate' instead. template[/tmp/vagrant-puppet-1/modules-0/logrotate/templates/conf.erb]:9
   (at /tmp/vagrant-puppet-1/modules-0/logrotate/templates/conf.erb:9:in `block in result')
Warning: Variable access via 'postrotate' is deprecated. Use '@postrotate' instead. template[/tmp/vagrant-puppet-1/modules-0/logrotate/templates/conf.erb]:14
   (at /tmp/vagrant-puppet-1/modules-0/logrotate/templates/conf.erb:14:in `block in result')
```