code: def get_key_value_pairs(self, subsystem, filename):
          
          assert subsystem in self
          return util.read_key_value_pairs_from_file(self.per_subsystem[subsystem], subsystem + '.' + filename)
docs: Read the lines of the given file from the given subsystem
          and split the lines into key-value pairs.
          Do not include the subsystem name in the option name.
          Only call this method if the given subsystem is available.