Add additional info. to image.delete notification
    
    Add additional information to the 'image.delete' notification payload
    to simplify external usage tracking systems.
    
    Please note that this is not a backwards-compatible change as the
    old payload was simply a string consisting of the image id.  This
    converts the payload to a dictionary.
    
    Change-Id: I641b7de91fcc14dc3c81ec942aeb0acf5a63d128