Fix a bug where a millicious user can access or manipulate an alloc in a
namespace they don't have access to.  The allocation endpoints perform
ACL checks against the request namespace, not the allocation namespace,
and performs the allocation lookup independently from namespaces.

Here, we check that the requested can access the alloc namespace
regardless of the declared request namespace.

Ideally, we'd enforce that the declared request namespace matches
the actual allocation namespace.  Unfortunately, we haven't documented
alloc endpoints as namespaced functions; we suspect starting to enforce
this will be very disruptive and inappropriate for a nomad point
release.  As such, we maintain current behavior that doesn't require
passing the proper namespace in request.  A future major release may
start enforcing checking declared namespace.