speed up is_key_in_yaml

[wochinge]

Problem
We currently use the function is_key_in_yaml to check if a file is a nlu or story file. This reads and parses the entire yaml file just to see if a key is present.

This makes the training data reading slow because:

• we read the entire file although this information might be right in the beginning
• we parse the entire file although we don't need to

Proposed Solution:

• Something like this: https://github.com/RasaHQ/rasa/pull/7944/files#r575331363