Modify stack ID lookup to only use stacks:lookup

Currently when we lookup a stack ID (fully qualified name/id) via
stacks:lookup, we also follow the redirect to stacks:show, which
is not allowed for stack users.  We don't actually need to follow
the redirect, as we already have the required ID, so instead we
return the redirect response, so we can just allow all users to
do stacks:lookup in the policy.

This is useful for clients who want to call API actions which are
allowed for stack users, e.g resource:signal without having to know
both the name and ID of the stack.

Change-Id: I6f079cbc7ac3c680f15941668b4efbe4c3c5d60d
Partial-Bug: #1291097
Merge "Modify stack ID lookup to only use stacks:lookup"