integrate RulePolicy with YAML training data reader

[wochinge]

With the new RulePolicy we got stories and rules. They have to be distinguishable for the polices, so that the policies can decide what they use for training.

Rick did a quick and dirty hack for the old markdown format here: #5898

Things to do:

• make the new YAMLReader read rules as part of the rules section (please check with @degiz for that)
• make the rules distinguishable for the policies
  • I see multiple options for this one. E.g. we could add another function to TrainingDataImporter or we add a flag to the DialogueStateTrackers which affects less things