LUIS training data not found

[wochinge]

Based on: https://stackoverflow.com/questions/56726192/unable-to-train-luis-model-in-rasa-nlu

Problem:
When Rasa is trained it uses the function rasa/data.py::_is_nlu_file to check whether a file in the directory is actually a NLU file. The function _is_nlu_file currently only works for rasa data, but not for other formats (such das LUIS).