Filter out images based on the content_set
    
    Freshmaker queries Lightblue to get images containing affected RPMs
    installed from a particular content_set. At the same time Freshmaker
    asks to Lightblue also all the images with enabled the
    auto_rebuild_tags tag (when not enabled the rebuilds of images in
    this repository are disabled).
    This gets done only because the Lightblue query will be easier and
    cleaner this way. But because of that some images returned by that
    query will not have the correct content_sets, for this reason we
    need to filter out images based on the content_sets.
    
    Signed-off-by: Giulia Naponiello <gnaponie@redhat.com>