Gets values of 'column' in 'record' in 'table'.

        This method is corresponding to the following ovs-vsctl command::

            $ ovs-vsctl get TBL REC COL[:KEY]

def get_db_attribute(self, table, record, column, key=None):
        """
        Gets values of 'column' in 'record' in 'table'.

        This method is corresponding to the following ovs-vsctl command::

            $ ovs-vsctl get TBL REC COL[:KEY]
        """
        if key is not None:
            column = '%s:%s' % (column, key)
        command = ovs_vsctl.VSCtlCommand(
            'get', (table, record, column))
        self.run_command([command])
        if command.result:
            return command.result[0]
        return None