mock.assert_called_once() is not a valid method

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

Borrowed HACKING rule from Davanum Srinivas's nova patch to
prevent it from appearing again.

Change-Id: Idac1d3c89c07e13c9a209663f4e557fcb7547821
Closes-Bug: #1365751
Closes-Bug: #1300265