image list: add VO attribute

Image lists can contain a 'ad:vo' attribute, that it seems to be
optional. If it is set, add it to our image list object.
dispatchers: handle correctly private images

Image lists protected by a token must be kept private. Moreover, if an
image has a VO associated to them, that VO should be able to have access
to those images.
dispatchers: remove unused prefix flag
dispatchers: set prefix from image source

Now it is possible to set the prefix via the image list source.

Closes #15