placement: s/None/null/ in consumer conflict msg

The placement API had a message complaining that it was "expecting None"
which is python-speak when it should be JSON speak.  This change
corrects the message and adds a gabbi test for it.

Change-Id: Ibfb3d581588c8eb954a9faca19d9cb26d2c28861
Closes-Bug: #1778071

nova/api/openstack/placement/util.py

@@ -640,7 +640,7 @@ def ensure_consumer(ctx, consumer_uuid, project_id, user_id,
             if consumer_generation is not None:
                 raise webob.exc.HTTPConflict(
                     _('consumer generation conflict - '
-                      'expected None but got %s') % consumer_generation)
+                      'expected null but got %s') % consumer_generation)
         # No such consumer. This is common for new allocations. Create the
         # consumer record
         try:

nova/tests/functional/api/openstack/placement/gabbits/allocations-1.28.yaml

@@ -50,6 +50,21 @@ tests:
   response_strings:
     - JSON does not validate
 
+- name: new version gen is not null no existing
+  PUT: /allocations/22222222-2222-2222-2222-222222222222
+  data:
+      allocations:
+          $ENVIRON['RP_UUID']:
+            resources:
+              DISK_GB: 10
+      project_id: $ENVIRON['PROJECT_ID']
+      user_id: $ENVIRON['USER_ID']
+      consumer_generation: 5
+  status: 409
+  response_strings:
+    - consumer generation conflict
+    - expected null but got 5
+
 - name: new version gen is None no existing
   PUT: /allocations/22222222-2222-2222-2222-222222222222
   data: