Returns a list of clusters in OpenNebula.

    .. versionadded:: 2016.3.0

    CLI Example:

    .. code-block:: bash

        salt-cloud -f list_clusters opennebula

def list_clusters(call=None):
    '''
    Returns a list of clusters in OpenNebula.

    .. versionadded:: 2016.3.0

    CLI Example:

    .. code-block:: bash

        salt-cloud -f list_clusters opennebula
    '''
    if call == 'action':
        raise SaltCloudSystemExit(
            'The list_clusters function must be called with -f or --function.'
        )

    server, user, password = _get_xml_rpc()
    auth = ':'.join([user, password])
    cluster_pool = server.one.clusterpool.info(auth)[1]

    clusters = {}
    for cluster in _get_xml(cluster_pool):
        clusters[cluster.find('NAME').text] = _xml_to_dict(cluster)

    return clusters