Overload urls and make list of lists of urls.

def normalize_urls(urls):
    """Overload urls and make list of lists of urls."""
    _urls = []
    if isinstance(urls, list):
        if urls:
            if isinstance(urls[0], list):
                # multiple connections (list of the lists)
                _urls = urls
            elif isinstance(urls[0], str):
                # single connections (make it list of the lists)
                _urls = [urls]
        else:
            raise RuntimeError("No target host url provided.")
    elif isinstance(urls, str):
        _urls = [[urls]]
    return _urls