[placement] Require at least one resource class in allocation

If an allocation was made with an empty resources object, a 500 response
code would result. This change adjusts the schema to use minProperties
of 1 to say there must be at least one resource class and value pair in
the allocation. If there is not a 400 response is returned.

As this is fixing 500 response to a useful error, no microversion is
required. A previous gabbi file which demonstrated the problem has been
updated to demonstrate that the problem has been fixed.

Change-Id: I7d9c64c77586564fb3bdbe92c693bd2a1bc06f24
Closes-Bug: #1714072

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

@@ -51,6 +51,7 @@ ALLOCATION_SCHEMA = {
                     },
                     "resources": {
                         "type": "object",
+                        "minProperties": 1,
                         "patternProperties": {
                             "^[0-9A-Z_]+$": {
                                 "type": "integer",

nova/tests/functional/api/openstack/placement/gabbits/allocations-bug-1714072.yaml

@@ -1,8 +1,7 @@
 # Bug 1714072 describes a situation where a resource provider is present in the
 # body of an allocation, but the resources object is empty. There should be at
 # least one resource class and value pair. If there is not a 400 response
-# should be returned. This gabbit file demonstrates the problem: a 500 on the final
-# test.
+# should be returned.
 
 fixtures:
     - APIFixture
@@ -59,5 +58,6 @@ tests:
             resources: {}
       project_id: 42a32c07-3eeb-4401-9373-68a8cdca6784
       user_id: 66cb2f29-c86d-47c3-8af5-69ae7b778c70
-   # This should be 400
-  status: 500
+  status: 400
+  response_strings:
+      - does not have enough properties