Fix docker_label keyword and fix label caps
    
    Previously the key 'docker_labels' was used but this isn't actually
    picked up by imagefactory because the parameter key it's looking for
    is 'docker_label'. This patch resolves that.
    
    Also included in this patch is a change to set the labels to be
    lowercase as defined in the Fedora Container Guidelines.
    
    Signed-off-by: Adam Miller <maxamillion@fedoraproject.org>