Make policy.json not filesystem location specific.
Make policy.json not filesystem location specific.

The interaction between the test suite and the policy.json file
is problematic, because it imposes an operational assumption that
the tests are running with the curdir set to nova/tests. There is
no other reason to need to be located in that directory.

Part of blueprint grizzly-testtools.

Change-Id: I863dac815a768a78fa84d632e5f77ed0f4f599b2

citrix-openstack-build/oslo.versionedobjects,openstack/oslo.versionedobjects