image list: add method to get list of subscribed images

Instead of making the comparison in other parts of the code it seems
reasonable to provide a method to get a list of the subscribed images,
if any.
dispatchers: add initial skeleton

Once the images have been downloaded it should be possible to dispatch
the images to one or several dispatchers. This way we should be able to
upload the images to an image service such as glance and, for example,
archive them for later inspection to a "file" dispatcher.

This commit provides the initial structure for this, together with a
"noop" dispatcher that will do nothing.