The 'model_update' helper has been added.

* it's now used to make cluster status changes easier
* it'll be potentially used to set cluster status + status description
  in the following way:
  context.model_update(obj, status='X', status_description='XXX')

Change-Id: I656f3f94b3dce697e060ed2ca2d4ad8f5c955794
Merge "The 'model_update' helper has been added."