Migrate to override_role for volume module (part 4)

Now that override_role has supplanted switch_role (which has
been deprecated) in [0], the RBAC tests need to switch to use
override_role.

This PS switches to override_role for the volume module. This
PS handles the remaining module for volume.

This PS also removes unnecessary indexing into response bodies.

[0] I670fba358bf321eae0d22d18cea6d2f530f00716

Partially Implements: blueprint rbac-utils-contextmanager

Change-Id: I3d134db7efb0717546c224c8e53ff687984d47d1
diff --git a/patrole_tempest_plugin/tests/api/volume/test_volume_services_rbac.py b/patrole_tempest_plugin/tests/api/volume/test_volume_services_rbac.py
index d36fb5a..1711c88 100644
--- a/patrole_tempest_plugin/tests/api/volume/test_volume_services_rbac.py
+++ b/patrole_tempest_plugin/tests/api/volume/test_volume_services_rbac.py
@@ -43,5 +43,5 @@
         service="cinder",
         rule="volume_extension:services:index")
     def test_list_services(self):
-        self.rbac_utils.switch_role(self, toggle_rbac_role=True)
-        self.services_client.list_services()['services']
+        with self.rbac_utils.override_role(self):
+            self.services_client.list_services()['services']