Hiera backend

    Allow the zone files to be retrieved from hiera-file, but default
    to the puppet templates directory.
    
    When creating a zone, add templatestorage => 'hiera'
    to make the module look up into hiera, otherwise, the default
    is 'puppet'.

This would still require us to pass in $template from the resource though, right?

```
nsd::zone { 'intern':
  template => 'intern.zone'
}
```
Which would them take be passed to hiera, so the hiera call would effectively be `hiera('intern.zone')`, but I'm not sure this is the desired outcome.