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'.

The full config for a zone, when using hiera would look like:
nsd::zone { 'intern':
  template => 'intern.zone',
  templatestorage => 'hiera'
}

then I for example have a intern.zone file as example: /etc/puppet/hieradata/${SUBDIR}/${SUBDIR}.d/intern.zone

then with hiera-file available, the hiera($template) would pick up that file from hiera, instead of the template subdirectory within the module.