E2E actions should result in BotUtterances

[wochinge]

With Rasa Open Source 2.0 we introduce end-to-end actions. End-to-end actions don't have a label (like utter_greet) but contain the actual user utterance, e.g.:

stories:
- story: My end-to-end story
  steps:
  - intent: greet
  - bot: Hey, how are you? 

Todos:

• make sure that an executed Action which has an end-to-end bot utterance, creates a BotUttered event
• think about how to distinguish "End-to-end actions" from regular actions in the domain