List configured volumes

    CLI Example:

    .. code-block:: bash

        salt '*' glusterfs.list_volumes

def list_volumes():
    '''
    List configured volumes

    CLI Example:

    .. code-block:: bash

        salt '*' glusterfs.list_volumes
    '''

    root = _gluster_xml('volume list')
    if not _gluster_ok(root):
        return None
    results = [x.text for x in _iter(root, 'volume')]
    return results