The current DB scoping (and service.py check) enforces retrieval of
stacks only when they match the request context project/tenant, but
with stack_user_project_id we must make an exception, since these
users will require access to the stack for a subset of operations,
which ones will be enforced via policy.json (not the engine)
Change-Id: I82bee14ae96e30653702b4bdb104a2b74cc20cd4
blueprint: instance-users
fef832fb2a