Run "gsutil" through "vpython". This ensures that the package set
accompanying it is completely controlled. This is notably important on
GCE instances, which "gsutil" expects to supply a
"google_compute_engine" package.

BUG=None
TEST=local
  - Ran `gsutil cp` on 64-bit Linux machine, seems to work.
  - Ran `gsutil cp` on GCE instance, seems to work.

Change-Id: Id771bc4da4de22c941b0ace4031cb0f3364feeeb
Reviewed-on: https://chromium-review.googlesource.com/527277Reviewed-by: Nodir Turakulov <nodir@chromium.org>
Reviewed-by: Robbie Iannucci <iannucci@chromium.org>
Commit-Queue: Daniel Jacques <dnj@chromium.org>