Add policy_dirs conf fixture

Add policy_dirs conf fixture

Unit tests that call into the policy enforcer from openstack
common will try to load files from the default directory of
'policy.d'.

Set the policy_dirs option to [] for unit tests since tests
are not trying to use policy from a directory like this, and
this will prevent test failures.

Change-Id: I7645179aeab3713a5addfccb0bb44a28e8fc4e87
Closes-Bug: #1412526