Use dogpile.cache to cache the listTaggedRPMS calls if possible
    
    If the same tag is queried with the same event, Pungi can cache the
    response and call the API again. Particularly for small composes this
    can save up significant amount of time.
    
    Merges: https://pagure.io/pungi/pull-request/1022
    Signed-off-by: Jan Kaluza <jkaluza@redhat.com>