Allow adding data images as volumes-from source

[svensp]

Use-Case: a plugin, tls certificate or configuration file has an existing docker image. The content of this image is needed in an app deployed via rancherize.

Concept:
New key data-images.

For each image contained here a sidekick is created for all services deployed by the app. All services and sidekicks receive a volumes-from to their respective container.