Support Dino's imagecollections with new workflow and butler

[drewoldag]

Dino's ImageCollections require a butler to retrieve the image files when creating a workunit. We currently don't support that because we've only been working with Colin's URI list that didn't really require a butler.

Additionally, starting from an image collection means that the first step in the workflow will be different. i.e. we start with an imagecollection instead of a URI list.