Fix the usage of tempest.client.Manager class

This should fix our gate. We should continue the work to remove
the dependencies from any module that isn't at tempest.lib.

Change-Id: I2158d1971a4187171a89169c3f324453f0ec13be
diff --git a/keystone_tempest_plugin/tests/base.py b/keystone_tempest_plugin/tests/base.py
index 83f5c12..6ba6659 100644
--- a/keystone_tempest_plugin/tests/base.py
+++ b/keystone_tempest_plugin/tests/base.py
@@ -32,7 +32,7 @@
         super(BaseIdentityTest, cls).setup_clients()
         credentials = common_creds.get_configured_admin_credentials(
             cls.credential_type, identity_version=cls.identity_version)
-        cls.keystone_manager = clients.Manager(credentials=credentials)
+        cls.keystone_manager = clients.Manager(credentials)
         cls.auth_client = cls.keystone_manager.auth_client
         cls.idps_client = cls.keystone_manager.identity_providers_client
         cls.mappings_client = cls.keystone_manager.mapping_rules_client