Unscoped List Stacks

This allows non tenant-scoped list of stacks for the management api
blueprint. A user allowed by the policy stacks:global_index will be
able to make a call to "/:tenant_id/stacks?global_tenant=true" and get
all stacks in Heat, and not just the ones owned by the user,
effectively bypassing tenant scoping.

If the user is not authorized by the policy and they pass the
global_tenant param, the request will get denied by the policy enforcer
and 403 Forbidden is returned.

Co-Authored-By: Richard Lee <rblee88@gmail.com>
Implements: blueprint management-api
Change-Id: I973a7ad2726eb6d9c972d08abe15e241aa120ec3
Merge "Unscoped List Stacks"