Fix test_show_server_usage not checking expected attributes

While os_compute_api:os-server-usage policy in Nova is deprecated [0]
the current test in Patrole should be fixed since it is not
checking for expected attributes which are conditionally injected
following successful policy authorization. See [0] for details
on expected attributes.

[0] https://github.com/openstack/nova/blob/15f1caf98a46ba0ab3f8365075c564e89f06eef3/nova/policies/server_usage.py#L32

Change-Id: Ibc632cc084c2edb58c336f5ff56d2902bf2ccc96
diff --git a/patrole_tempest_plugin/tests/api/compute/test_server_misc_policy_actions_rbac.py b/patrole_tempest_plugin/tests/api/compute/test_server_misc_policy_actions_rbac.py
index 9f627fc..c6f2fc4 100644
--- a/patrole_tempest_plugin/tests/api/compute/test_server_misc_policy_actions_rbac.py
+++ b/patrole_tempest_plugin/tests/api/compute/test_server_misc_policy_actions_rbac.py
@@ -472,10 +472,18 @@
         """Test show server usage, part of os-server-usage.
 
         TODO(felipemonteiro): Once multiple policy testing is supported, this
-        test can be combined with the generic test for showing a server.
+        test should also check for additional policies mentioned here:
+        https://github.com/openstack/nova/blob/master/nova/policies/server_usage.py
         """
+        expected_attrs = ('OS-SRV-USG:launched_at',
+                          'OS-SRV-USG:terminated_at')
+
         with self.rbac_utils.override_role(self):
-            self.servers_client.show_server(self.server['id'])
+            body = self.servers_client.show_server(self.server['id'])['server']
+        for expected_attr in expected_attrs:
+            if expected_attr not in body:
+                raise rbac_exceptions.RbacMalformedResponse(
+                    attribute=expected_attr)
 
     @utils.requires_ext(extension='os-simple-tenant-usage', service='compute')
     @rbac_rule_validation.action(