Mock: Fixed partition.start/stop() result.
Mock: Fixed partition.start/stop() result.

Details:
- The result of Partition.start/stop() with wait_for_completion=True
  against a faked partition was 'None' so far. That is not
  consistent with the HMC API, where an empty dictionary is returned
  in the completed async job result.
  Fixed the URI handler so that it now returns an empty dictionary.
- The unit test cases have not been adjusted because they test with
  wait_for_completion=False only.

Signed-off-by: Andreas Maier <38dbb04957d222ea6ce72de72413b1e55e665472@de.ibm.com>

zhmcclient/python-zhmcclient,zhmcclient/python-zhmcclient