remove unused option

Why was this here?
image_list: split into several files
cache: split out into its own module

Comment it out from the commands until it is working again
image list manager: code cleanup
cache: enable cache again on its own module
image: do not use class attributes.
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.
start using oslo.log

Instead of use logging, we leverage it in oslo.log. It is not yet
released, but it will be soon, so we can start using it right now.
dispatchers: add intial glance dispatcher

This dispatcher will upload images to glance with the relevant
metadata. Currently it does not support the removal of images (WIP).

closes #2
dispatchers: add support for passing extra metadata

When we are dispatching an image sometimes it is needed to pass extra
metadata to the image (as the image list).
dispatchers: pass down the image list name to the dispatchers

The dispatcher manager will pass down the image list name to each of the
dispatchers, so that an image can be  associated to the an image list.
dispatchers: add sync method to dispatchers

This method should sync the dispatched data with the data being
presented by the image list. Any image not present, should be deleted.
dispatchers: glance - add sync method

Look for the images associated with an image list, then delete the
images that are not valid anymore.

closes #2