Add policy support

Using oslo.policy to support access control for Distil API
access, which will follow the community way.

Change-Id: I670d9fde4f5c368e82c26b512b7c1e46c2e380ec

etc/policy.json.sample

@@ -0,0 +1,8 @@
+{
+    "context_is_admin": "role:admin",
+    "admin_or_owner": "is_admin:True or project_id:%(project_id)s",
+    "default": "rule:admin_or_owner",
+
+    "rating:costs:get": "rule:context_is_admin",
+    "rating:usages:get": "rule:context_is_admin",
+}