docs: def _get_roster(self):
          '''
          Read roster filename as a key to the data.
          :return:
          '''
          roster_file = salt.roster.get_roster_file(self.opts)
          if roster_file not in self.__parsed_rosters:
              roster_data = compile_template(roster_file, salt.loader.render(self.opts, {}),
                                             self.opts['renderer'], self.opts['renderer_blacklist'],
                                             self.opts['renderer_whitelist'])
              self.__parsed_rosters[roster_file] = roster_data
          return roster_file