Returns a connection to a mongo-clusters.

        Args:
            label (string): the label of a cluster.

        Returns:
            A connection to the cluster labeld with label.

        Raises:
            AttributeError: there is no cluster with the given label in the
                config

def get_cluster(self, label):
        """Returns a connection to a mongo-clusters.

        Args:
            label (string): the label of a cluster.

        Returns:
            A connection to the cluster labeld with label.

        Raises:
            AttributeError: there is no cluster with the given label in the
                config
        """
        for cluster in self._clusters:
            if label == cluster['label']:
                return self._get_connection(cluster)
        raise AttributeError('No such cluster %s.' % label)