Introduce a skip_validation parameter in yaml reader

[kalkbrennerei]

Description of Problem:
Currently yaml_story_reader::read_from_string always validates it's input.
This makes reading strings from the database (that were already validated) very slow, which e.g. happens in requests to GET /domainWarnings in rasa-x.

Overview of the Solution:
To speed up reading strings from the database we introduce a skip_validation parameter in yaml_story_reader::read_from_string and yaml_story_reader::read_from_file

Examples (if relevant):

Blockers (if relevant):

Definition of Done:

[kalkbrennerei]

rasa-x issue for context