Link to index.html in log indexes

The index.html files are not automatically served by GCS, so update
the links we generate to point directly to them.

Change-Id: Icaed78ba1513ca1587d7e582cf9b02939d638e7c

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

diff --git a/roles/upload-logs-gcs/library/zuul_google_storage_upload.py b/roles/upload-logs-gcs/library/zuul_google_storage_upload.py
index e5a2143..774a786 100755
--- a/roles/upload-logs-gcs/library/zuul_google_storage_upload.py
+++ b/roles/upload-logs-gcs/library/zuul_google_storage_upload.py
@@ -465,10 +465,12 @@
                                        file_details.filename),
                 }))
             filename = file_details.filename
+            link_filename = filename
             if file_details.folder:
                 filename += '/'
+                link_filename += '/index.html'
             output += '<td><a href="%s">%s</a></td>' % (
-                urlparse.quote(filename),
+                urlparse.quote(link_filename),
                 filename)
             output += '<td>%s</td>' % time.asctime(
                 file_details.last_modified)