Fix GCS log upload

The google-auth Credential class added some new variables in the
constructor; make sure that we call it in order to avoid later
reference errors.

Change-Id: I338a3368b90a6612801ba587982d3053bbd3ba78

roles/upload-logs-gcs/library/zuul_google_storage_upload.py

@@ -260,6 +260,7 @@ def sizeof_fmt(num, suffix='B'):
 
 class Credentials(gce_cred.Credentials):
     def __init__(self, path):
+        super().__init__()
         self._path = path
         self.refresh(None)