(PUP-3462) Add the key 'calling_class_path' which is 'calling_class' with '::' by '/'

This enables to have the class hiera yaml files in directories and enables to
have class based hiera data on windows which does not allow ":" in path names.
Merge pull request #3175 from vinzent/calling_class_path

(PUP-3462) Hiera scope: add the key 'calling_class_path'