http2/h2demo: set externalTrafficPolicy to preserve client IPs

So https://http2.golang.org/reqinfo has a RemoteAddr of the client's
actual IP address.

Change-Id: I9db52b2f451ad937d013dd78078e4228e732ffe0
Reviewed-on: https://go-review.googlesource.com/99235
Reviewed-by: David Anderson <danderson@google.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

http2/h2demo/service.yaml

@@ -3,6 +3,7 @@ kind: Service
 metadata:
   name: h2demo
 spec:
+  externalTrafficPolicy: Local
   ports:
     - port: 80
       targetPort: 80