Mistake was introduced in https://review.openstack.org/#/c/554950
Added an abstract method list to the base ResourceManager that
raises a notImplementedError if called (instead of using
abc.abstractmethod that would force the implementation of a list
method that might not be required for all resources)
Closes-Bug: #1820045
Change-Id: I0c69e1beeb452a95cc5988a462f733806afbfafc