cells: don't pass context to instance.save in instance_update_from_api

Commit 1cf0890fdd creates a wedge whenever
someone tries to pass a context to a remotable nova object method, like
instance.save().  There was still an occurrence of this happening in
cells so let's clean it up and make cells happy(ish) again.

Closes-Bug: #1431562

Change-Id: I58e626cb37fe4d34010f6b881f06f0d10f1e6e78

nova/cells/messaging.py

@@ -828,7 +828,7 @@ class _TargetedMessageMethods(_BaseMessageMethods):
         # NOTE(alaski): A cell should be authoritative for its system_metadata
         # and metadata so we don't want to sync it down from the api.
         instance.obj_reset_changes(['metadata', 'system_metadata'])
-        instance.save(message.ctxt, expected_vm_state=expected_vm_state,
+        instance.save(expected_vm_state=expected_vm_state,
                       expected_task_state=expected_task_state)
 
     def _call_compute_api_with_obj(self, ctxt, instance, method, *args,

nova/tests/unit/cells/test_cells_messaging.py

@@ -1205,7 +1205,7 @@ class CellsTargetedMethodsTestCase(test.TestCase):
                 self.assertEqual(set(['user_data']),
                                  instance.obj_what_changed())
 
-        instance.save(self.ctxt, expected_task_state='exp_task',
+        instance.save(expected_task_state='exp_task',
                       expected_vm_state='exp_vm').WithSideEffects(
                               _check_object)