Parses a --filter-tag argument

def filter_tag(arg):
    """ Parses a --filter-tag argument """
    try:
        strip_len = len('Key=')
        key, value = arg[strip_len:].split(',Value=', 1)
        return key, value
    except:
        msg = 'Invalid --filter-tag argument: {}'
        raise argparse.ArgumentTypeError(msg.format(arg))