heat api : Add policy.json authorization to CFN API

Adds a basic policy.json to authorize all actions for the CFN API -
this will deny access to the in-instance users defined in stack
templates (which are assigned the heat_stack_user role) to all API
actions apart from DescribeStackResource, which is used for metadata
updates

ref bug 1115758

Change-Id: I1431c1f23593fffd0f911f71ef9c186a43e5063a
Signed-off-by: Steven Hardy <shardy@redhat.com>
Merge "heat api : Add policy.json authorization to CFN API"