[placement] Add gabbi coverage for inv of missing rp

There was no gabbi coverage for the case where a GET request was being
made for single class of inventory on a non-existent resource provider.
This adds it.

Note that it would have been possible to add an error code/comment for
the raised exception, but this was not done in this change because there
is another change in progress which is adding the appropriate code.

Change-Id: I1858f4eb617cc81b41610195bb9a33a9059850f7

nova/tests/functional/api/openstack/placement/gabbits/inventory.yaml

@@ -418,6 +418,10 @@ tests:
   response_json_paths:
       $.errors[0].title: Not Found
 
+- name: get missing resource provider inventory
+  GET: /resource_providers/2e1dda56-8b18-4fb9-8c5c-3125891b7143/inventories/VCPU
+  status: 404
+
 - name: create another resource provider
   POST: /resource_providers
   request_headers: