policy.json allow all users to do stacks:lookup

We should allow all users, including stack users to do a stacks:lookup
otherwise they need to know both the stack name and ID to make a
request.  This allows us to ensure that any authenticated user can
make a request e.g via heatclient, if they can provide a valid name
or stack ID which they have access to.

Change-Id: Iff992dc5026f436eb22f437ea39b9511ed7fe25d
Partial-Bug: #1291097
Merge "policy.json allow all users to do stacks:lookup"