Call clear_isolated_creds in scenario tests

After commit b7144eb08175d010e1300e14f4f75d04d9c63c98, tempest.scenario.manager
doesn't clear isolated credentials anymore. This patch adds a tearDownClass
method to class OfficialClientTest so isolated credentials can be cleared.

Change-Id: Ic03e0d5026e09bff6d1d20daf2ee72bfdb106145
Closes-Bug: #1343027
diff --git a/tempest/scenario/manager.py b/tempest/scenario/manager.py
index ca79325..0b3ad1f 100644
--- a/tempest/scenario/manager.py
+++ b/tempest/scenario/manager.py
@@ -87,6 +87,11 @@
         cls.ceilometer_client = cls.manager.ceilometer_client
 
     @classmethod
+    def tearDownClass(cls):
+        cls.isolated_creds.clear_isolated_creds()
+        super(OfficialClientTest, cls).tearDownClass()
+
+    @classmethod
     def _get_credentials(cls, get_creds, ctype):
         if CONF.compute.allow_tenant_isolation:
             creds = get_creds()