Retry: fix the Python invocation for cros_chromite

Retry: fix the Python invocation for cros_chromite

Partial reimplementation of reverted
https://codereview.chromium.org/890803003/

This fixes the Python invocation part. The runhooks invocation
(recursion) part that caused the other CL to be reverted will be
addressed elsewhere.

BUG=414436
TEST=local
  - Ran `gclient sync`, works.
  - Ran `gclient runhooks`, works.

Committed: http://src.chromium.org/viewvc/chrome?view=rev&revision=293885

[dnj, 2015-01-30 20:29:32]

dnj@chromium.org changed reviewers:
+ agable@chromium.org, petermayo@chromium.org

[dnj, 2015-01-30 20:29:32]

PTAL, shouldn't break anything this time :)

[Peter Mayo, 2015-01-30 20:43:40]

LGTM - wish you had re-uploaded the original as the first patchset

[dnj, 2015-01-30 20:44:46]

The CQ bit was checked by dnj@chromium.org

[commit-bot: I haz the power, 2015-01-30 20:45:31]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/888113002/1

[commit-bot: I haz the power, 2015-01-30 20:46:45]

Committed patchset #1 (id:1) as
http://src.chromium.org/viewvc/chrome?view=rev&revision=293885