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=chromium:741001
TEST=local
  - Ran `gsutil cp` on 64-bit Linux machine, seems to work.
  - Ran `gsutil cp` on GCE instance, seems to work.

This is a reland of 23836a64.

Change-Id: If87261abd505ef77f95b3c70b6bd23f3e798b41d
Reviewed-on: https://chromium-review.googlesource.com/656565Reviewed-by: Nodir Turakulov <nodir@chromium.org>
Reviewed-by: Robbie Iannucci <iannucci@chromium.org>
Commit-Queue: Daniel Jacques <dnj@chromium.org>