Allow an interactive to reference a model with different ids

This fix the bug where interactives that have more than one reference to a model are failing. PT Story https://www.pivotaltracker.com/s/projects/731349/stories/46307653

Some interactives, "Replacing Bitmap Images with SVG", have more than one model reference for a specific model.

For example, the "Replacing Bitmap Images with SVG" at http://lab.dev.concord.org/examples/interactives/interactives.html#interactives/layout-tests/svg-images.json 
Has four different references, (png, svg1, svg2, svg3), to the same model.

This can be seen at this URL by opening the "Interactive Browser" and looking at this interactive's models