Currently in LUCI `git cl` is invoked on chromium_presubmit using vpython. This
means that it operates under the depot_tools .vpython environment (which is
blank).

Some features in presubmit_support allow invocation of python subprocesses, and
previously they would use `sys.executable` (especially on windows, but
occasionally on non-windows as well).

In non-vpython environments, this just picks up the system python and whatever
modules happen to be installed there. Some python unittests work around this on
non-windows systems by having a shebang line which explicitly invokes vpython.

This CL changes presubmit_support so that invocations of 'python', or executions
of '.py' scripts will always end up invoking vpython. In the best case, this will
pick up the invoked scripts' vpython environment. In the worst case, this will
invoke the script under an empty vpython environment (aka "stock python").

R=dpranke@chromium.org, jbudorick@chromium.org, tandrii@chromium.org, tikuta@chromium.org

Bug: 821669
Change-Id: I5d2d5dfd0364022d56833c2c8af4983553a29c7a
Reviewed-on: https://chromium-review.googlesource.com/961865Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Reviewed-by: Andrii Shyshkalov <tandrii@chromium.org>
Commit-Queue: Robbie Iannucci <iannucci@chromium.org>