policy: rename Exception to avoid nose regression
policy: rename Exception to avoid nose regression

The name TestException makes nose think that this is a test case, so it
fails with nose 1.3.1. Renaming the exception thinks that problem.

Change-Id: I50b760ec24a67900f80d961f738d5fd7643561bc

openstack/oslo.policy,rodrigods/oslo.policy,citrix-openstack-build/oslo.policy