text: Title: Re-usable stages are working incorrectly
  Body: When trying to use a re-usable stage. I get this intermittent issue from 
  Tavern saying it cant find the start mark (AttributeError: 'dict' object has no attribute 'start_mark'
  ). I have multiple stages in different files. It may also be helpful to users to put how to properly use the re-usable stage feature in the documentation. 