Read roster filename as a key to the data.
        :return:

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