fix: miss attr _discovery_cache in ServiceTokenAuthWrapper

Add the super method to the ServiceTokenAuthWrapper class
to get the _discovery_cache attribute of the parent class.

the error info is below while neutron is authenticated by
keystoneauth plug in task inspector enroll baremetal node:
ERROR oslo_messaging.rpc.server:
Exception during message handling: AttributeError:
'ServiceTokenAuthWrapper' object has no attribute '_discovery_cache'

Change-Id: Icc7c4e25a123b5565c94f43f932ee32f9f304a76

keystoneauth1/service_token.py

@@ -20,6 +20,7 @@ __all__ = ('ServiceTokenAuthWrapper',)
 class ServiceTokenAuthWrapper(plugin.BaseAuthPlugin):
 
     def __init__(self, user_auth, service_auth):
+        super(ServiceTokenAuthWrapper, self).__init__()
         self.user_auth = user_auth
         self.service_auth = service_auth