[autotest] change python shebang to python2

Make sure all these scripts run as Python 2 since there is no support
for Python 3 in this code base.  This allows us to change the default
`python` to Python 3.

BUG=chromium:990593
TEST=CQ passes

Exempt-From-Owner-Approval: everyone has chimed in
Change-Id: I05321d92696f4550ced0741ead5023c8ec43c1e7
Reviewed-on: https://chromium-review.googlesource.com/1734588
Tested-by: Mike Frysinger <vapier@chromium.org>
Commit-Ready: ChromeOS CL Exonerator Bot <chromiumos-cl-exonerator@appspot.gserviceaccount.com>
Legacy-Commit-Queue: Commit Bot <commit-bot@chromium.org>
Reviewed-by: Vadim Bendebury <vbendeb@chromium.org>
Reviewed-by: Allen Li <ayatane@chromium.org>
diff --git a/frontend/manage.py b/frontend/manage.py
index 8ae4f17..7f0e6af 100755
--- a/frontend/manage.py
+++ b/frontend/manage.py
@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python2
 import sys
 import setup_django_environment