Use immutable roles in tests

Without this patch, bootstrap emits deprecation warnings in the tests
about opting into immutable roles. This change follows the warnings'
advice so that the warnings no longer clutter up the log output.

Change-Id: I72acef2409acc5efad84390cd7f63683a9842a5a

keystone/tests/unit/base_classes.py

@@ -67,6 +67,7 @@ class TestCaseWithBootstrap(core.BaseTestCase):
         self.bootstrapper.admin_role_name = 'admin'
         self.bootstrapper.service_name = 'keystone'
         self.bootstrapper.public_url = 'http://localhost/identity/'
+        self.bootstrapper.immutable_roles = True
 
         try:
             PROVIDERS.resource_api.create_domain(