'Return a list of cluster information in the current zone or all zones.
:keyword  ex_zone:  Optional zone name or None
:type     ex_zone:  ``str`` or :class:`GCEZone` or
:class:`NodeLocation` or ``None``'

def list_clusters(self, ex_zone=None):

    request = ('/zones/%s/clusters' % ex_zone)
    if (ex_zone is None):
        request = '/zones/clusters'
    response = self.connection.request(request, method='GET').object
    return response
