dashboard/cmd/buildlet: support for writing files from tgz URL directly

Client + server support, and gomote flags.

Change-Id: I91320f47731f8c69b84c4961028bfbbdfc85467a
Reviewed-on: https://go-review.googlesource.com/3029
Reviewed-by: Andrew Gerrand <adg@golang.org>
3 files changed