Lists all client labels known to the system.

def ReadAllClientLabels(self):
    """Lists all client labels known to the system."""
    result = set()
    for labels_dict in itervalues(self.labels):
      for owner, names in iteritems(labels_dict):
        for name in names:
          result.add(rdf_objects.ClientLabel(owner=owner, name=name))

    return list(result)