Fix wrong method name assert_called_once
Fix wrong method name assert_called_once

There isn't method assert_called_once, and it doesn't really
check anything. We'd better check mock.called directly.

Change-Id: Iffbaa176c3efd41084a655adcbbb0821d8ad496d

enzochiau/cliff,enzochiau/cliff,openstack/cliff,idjaw/cliff,dtroyer/cliff,idjaw/cliff,openstack/cliff,dtroyer/cliff