Add oslo policy test

The policy option is configured using oslo mudule,
but no related test, so add it.

Change-Id: I18c6e9a3d07bbe85a8649e6d6f8e6f256ebb9a18

spec/classes/nova_policy_spec.rb

@@ -20,6 +20,7 @@ describe 'nova::policy' do
         :key   => 'context_is_admin',
         :value => 'foo:bar'
       })
+      is_expected.to contain_nova_config('oslo_policy/policy_file').with_value('/etc/nova/policy.json')
     end
   end