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

def get_value(self, subsystem, option):
        """
        Read the given value from 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, 'Subsystem {} is missing'.format(subsystem)
        return util.read_file(self.per_subsystem[subsystem], subsystem + '.' + option)