'Return a list of current token_id\'s for a user.
This is effectively a private method only used by the ``delete_tokens``
method and should not be called by anything outside of the
``token_api`` manager or the token driver itself.
:param user_id: identity of the user
:type user_id: string
:param tenant_id: identity of the tenant
:type tenant_id: string
:param trust_id: identity of the trust
:type trust_id: string
:param consumer_id: identity of the consumer
:type consumer_id: string
:returns: list of token_id\'s'

@abc.abstractmethod
def _list_tokens(self, user_id, tenant_id=None, trust_id=None, consumer_id=None):

    raise exception.NotImplemented()
