cros_set_lsb_release: Convert to python

Re-write the cros_set_lsb_release script in python.

BUG=brillo:1010
TEST=manual; built image before and after change, compared
     the lsb-release files of each
CQ-DEPEND=CL:272547

Change-Id: I4d34742ac7a5e56f5385030d656dbaf738d81978
Reviewed-on: https://chromium-review.googlesource.com/272517
Reviewed-by: Mike Frysinger <vapier@chromium.org>
Commit-Queue: Sheng-liang Song <ssl@chromium.org>
Commit-Queue: Steve Fung <stevefung@chromium.org>
Trybot-Ready: Steve Fung <stevefung@chromium.org>
Tested-by: Steve Fung <stevefung@chromium.org>
7 files changed