Canary test for os-traits version

Since [1] we have decided to pin placement to the latest versions of
os-resource-classes and os-traits. Turns out we had gabbi tests that
would fail as soon as the os-resource-classes upper constraint was
bumped, alerting us that it was time to sync that requirement. This
commit adds a similar test for os-traits.

[1] https://review.opendev.org/#/c/658419/

Change-Id: I57e834311c73282a2e7a00b14823812cb6f3e1e6

placement/tests/functional/gabbits/traits.yaml

@@ -9,6 +9,10 @@ defaults:
         openstack-api-version: placement 1.6
 
 tests:
+- name: what is at traits
+  GET: /traits
+  response_json_paths:
+      $.traits.`len`: 223  #  Number of standard traits
 
 - name: create a trait without custom namespace
   PUT: /traits/TRAIT_X