Write the given value for the given subsystem.
        Do not include the subsystem name in the option name.
        Only call this method if the given subsystem is available.

def set_value(self, subsystem, option, value):
        """
        Write the given value for the given subsystem.
        Do not include the subsystem name in the option name.
        Only call this method if the given subsystem is available.
        """
        assert subsystem in self
        util.write_file(str(value), self.per_subsystem[subsystem], subsystem + '.' + option)