Remove old syntax

We've been seeing these warnings for quite some time now.  Time to clear them up.

```
Warning: Variable access via 'backends' is deprecated. Use '@backends' instead. template[/vagrant/vendor/modules/hiera/templates/hiera.yaml.erb]:3
Warning: Variable access via 'hierarchy' is deprecated. Use '@hierarchy' instead. template[/vagrant/vendor/modules/hiera/templates/hiera.yaml.erb]:6
Warning: Variable access via 'datadir' is deprecated. Use '@datadir' instead. template[/vagrant/vendor/modules/hiera/templates/hiera.yaml.erb]:9
Warning: Variable access via 'extra_config' is deprecated. Use '@extra_config' instead. template[/vagrant/vendor/modules/hiera/templates/hiera.yaml.erb]:11

```