isolated creadentials are not cleaned up

Due to type introduced last weak, the isolated credentials are
not claned up.
In a neutron envoriements it can mean more than 1 Gigabyte memory used
in dnsmasq and in the neutron-ns-metadata-proxy.

Change-Id: I09c538ed1139380f8cd8a662e60d69d3f605cd67
diff --git a/tempest/test.py b/tempest/test.py
index d80e478..ee9cd57 100644
--- a/tempest/test.py
+++ b/tempest/test.py
@@ -486,7 +486,7 @@
         """
         Clears isolated creds if set
         """
-        if hasattr(cls, '_cred_provider'):
+        if hasattr(cls, '_creds_provider'):
             cls._creds_provider.clear_isolated_creds()
 
     @classmethod