Construct command line strings for a config server.

def _construct_config(self, basedir, port, name=None, isreplset=False):
        """Construct command line strings for a config server."""
        if isreplset:
            return self._construct_replset(basedir=basedir, portstart=port,
                                           name=name,
                                           num_nodes=list(range(
                                               self.args['config'])),
                                           arbiter=False, extra='--configsvr')
        else:
            datapath = self._create_paths(basedir, name)
            self._construct_mongod(os.path.join(datapath, 'db'),
                                   os.path.join(datapath, 'mongod.log'),
                                   port, replset=None, extra='--configsvr')