Refs #10450.  Added a new property to algorithm.

to override the log value from workspace.  and
a new unit test is added for the new feature.

On branch feature/10450_record_order_by_runnumber

      modified:   ../Mantid/Framework/PythonInterface/plugins/algorithms/ExportExperimentLog.py
      modified:   ../Mantid/Framework/PythonInterface/test/python/plugins/algorithms/ExportExperimentLogTest.py