Allow gather phase reuse on `product_id` change.
    
    I've analysed multiple nigthly composes and often the only difference
    in the configuration between two nightly composes is different
    `product_id` commit hash.
    
    The `product_id` is used in later `createrepo` phase and does not
    influence the gather phase at all. I therefore think it can be
    whitelisted in gather phase reuse code.
    
    Signed-off-by: Jan Kaluza <jkaluza@redhat.com>