Fix up Vary headers on 30x redirects from Apache

Apache on the application servers is not properly configured to send
Vary: X-Forwarded-Proto headers when sending redirects. Squid seems
to not cache the 30x responses, but Varnish does, and until we fix
the Apache configuration we need to fix it up manually.

bug: 51700

Change-Id: Ieaff3fdd4d67ea76c6b8b898272e4377be759854