Use form data, not query params, for the ISO POST for updates
    
    As noted at https://progress.opensuse.org/issues/119200 , we
    ran into a fun problem with a KDE update with 200+ packages in
    it: the request URI (with all the parameters encoded in it) was
    longer than the ~8k limit that's the default for Apache and
    also, apparently, something deep in Perl that caused openQA to
    barf on the request even when I changed Apache's config.
    
    It turns out, all the bits are already in place in openQA itself
    and openQA-python-client to let us send the parameters as form
    data rather than encoded in the URI. All we have to do is pass
    the dict as the `data` arg rather than the `params` arg. So,
    let's do that! This should hopefully mean we never have this
    problem again even if someone makes an even bigger update.
    
    Not doing the same (at least for the moment) for other queries
    as I don't think there's any chance of them producing URIs this
    long.
    
    This also drops some stray debug `print()` statements in the
    tests which I noticed while updating them.
    
    Signed-off-by: Adam Williamson <awilliam@redhat.com>