mock.assert_called_once() is not valid method

There are a few places in tests/conductor/test_manager.py where calls to
mock.assert_called_once() are made. The problem is that this isn't a valid
mock method, and 'assert_called_once' isn't a valid method of whatever is
being mocked. So it doesn't actually do anything.

This replaces the assert_called_once() calls with valid
assert_called_once_with() calls.

Change-Id: I7d46cf866bcce3b7e09bc556f2a1ddd10e4d1ca5
Partial-Bug: #1365745
Merge "mock.assert_called_once() is not valid method"