Fix improper prompt when update RC with existed one's name.

When update resource class with existed one's name,
the exception message was formatted by the updated resource
class's name, rather than the required existed one's name.
This patch formats the message with existed one's name, and
changing tests to add the right name.

Change-Id: I78ae8d872748de243d74b9954ce634fccf5e7310
Closes-Bug: #1663163

nova/api/openstack/placement/handlers/resource_class.py

@@ -183,7 +183,7 @@ def update_resource_class(req):
     except exception.ResourceClassExists:
         raise webob.exc.HTTPConflict(
             _('Resource class already exists: %(name)s') %
-            {'name': name},
+            {'name': rc.name},
             json_formatter=util.json_error_formatter)
     except exception.ResourceClassCannotUpdateStandard:
         raise webob.exc.HTTPBadRequest(

nova/tests/functional/api/openstack/placement/gabbits/resource-classes.yaml

@@ -151,6 +151,7 @@ tests:
   status: 409
   response_strings:
       - Resource class already exists
+      - $ENVIRON['CUSTOM_RES_CLASS']
 
 - name: update custom resource class
   PUT: /resource_classes/$ENVIRON['CUSTOM_RES_CLASS']