hiera.yaml: Migrate hiera to puppet 5

What changed:

• version is now 5
• define a default backend to yaml (data_hash key)
• path must be specified up to the filename extension so 'name' mostly became 'path'
• added the 'name' key to actually describe our entries

doc: https://puppet.com/docs/puppet/4.10/hiera_migrate_v4_yaml.html