There was a note in the API reference for "List allocations" indicating
the response payload when a nonexistent consumer is queried, but the
parameter table entry for `consumer_generation` was duplicated from the
"Manage" and "Update" operations, which was confusing.
This commit clarifies that the `consumer_generation` will be entirely
absent from the response payload in this scenario. It also adds the same
clarification for the `project_id` and `user_id` fields. And tightens up
the gabbi coverage proving same.
Change-Id: I70f22b246d0c0acd1ae87a75bc438a9258b704ad
Story: #2006726
Task: #37146
f3558482ab