mock.assert_called_once() is not a valid method

mock.assert_called_once() is a no-op that tests nothing.  Instead
with mock.assert_called_once_with() should be used (or use
assertEqual(1, mock_obj.call_count) if you don't want to check
parameters).

Add a new HACKING rule for nova to prevent assert_called_once()
usage from creeping in.

Closes-Bug: #1365751

Change-Id: I1055093a1c31792b6411a3cd46c80b8bcaaf78c1
Merge "mock.assert_called_once() is not a valid method"