'Self-service password change.
:param str user_id: User ID.
:param str new_password: New password.
:raises keystone.exception.UserNotFound: If the user doesn\'t exist.
:raises keystone.exception.PasswordValidation: If password fails
validation'

@abc.abstractmethod
def change_password(self, user_id, new_password):

    raise exception.NotImplemented()
