Update git submodules

Update git submodules

* Update docs/submodules/yardstick from branch 'master'
  - Merge "assertTrue(mock.called) -> mock.assert_called"
  - assertTrue(mock.called) -> mock.assert_called

    When checking whether a mocked method has been called,
    mock.assert_called or mock.assert_called_once should be
    used, and not unittest.assertTrue(mock.called)

    This change does this update.
    It can be verified with:
      grep -irn "self.assertTrue(.*called)" --exclude-dir=.tox
    And the replacement was done with:
      sed -i 's/self.assertTrue(\(.*\).called)/\1.assert_called_once()/g' <list_of_files>

    Change-Id: I4f26e0c736bf33e0b2413c8e8c33dbdb91f090e2
    JIRA: YARDSTICK-865
    Signed-off-by: Emma Foley <emma.l.foley@intel.com>