def get_blocked_usernames():
    """ get a list of blocked usernames from redis """
    if config.DISABLE_USERNAME_LOCKOUT:
        # There are no blocked usernames since we disabled them.
        return []
    key = get_username_blocked_cache_key("*")
    key_list = [redis_key.decode('utf-8')
                for redis_key in REDIS_SERVER.keys(key)]
    return strip_keys(key_list)
get a list of blocked usernames from redis