Get the ntp server port

def _get_service_port(self, instance):
        """
        Get the ntp server port
        """
        host = instance.get('host', DEFAULT_HOST)
        port = instance.get('port', DEFAULT_PORT)
        # default port is the name of the service but lookup would fail
        # if the /etc/services file is missing. In that case, fallback to numeric
        try:
            socket.getaddrinfo(host, port)
        except socket.gaierror:
            port = DEFAULT_PORT_NUM

        return port