'Update an existing group.
:param str group_id: Group ID.
:param dict group: Group modification. See group schema in
:class:`~.IdentityDriverBase`. Required properties cannot be
removed.
:returns: group, matching the group schema.
:rtype: dict
:raises keystone.exception.GroupNotFound: If the group doesn\'t exist.
:raises keystone.exception.Conflict: If a duplicate group exists.'

@abc.abstractmethod
def update_group(self, group_id, group):

    raise exception.NotImplemented()
