Add network prefix.

        Args:
            prefix (str): network prefix.
            flags (str): network prefix flags, please refer thread documentation for details
            prf (str): network prf, please refer thread documentation for details

def add_prefix(self, prefix, flags, prf):
        """Add network prefix.

        Args:
            prefix (str): network prefix.
            flags (str): network prefix flags, please refer thread documentation for details
            prf (str): network prf, please refer thread documentation for details
        """
        self._req('prefix add %s %s %s' % (prefix, flags, prf))
        time.sleep(1)
        self._req('netdataregister')