Goal:

• While creating a blueprint, optionally select a content template in image builder
  • if selected, the repositories step will show the repositories from the template, not allowing selecting
• Have this template be saved to blueprint and reloaded on edit
• On blueprint export, just export the repos and date information from the template (so template import isn't supported)
• When building an image using a template image-builder backend would just pass the snapshot list to composer.  Composer doesn't need to know about the content template for image building purposes