devapp/devappserver: support TLS for local dev and prod via autocert

Updates golang/go#20691

Change-Id: If0f160713496b1c9ebceacb4975e9afa44539e5a
Reviewed-on: https://go-review.googlesource.com/46720
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
4 files changed