Reland "Store metadata about updates in a file alongside with payload."

Relands CL: I3695b0903514eb6840e88810b8546fdca690819e

This original CL had an issue where we only generated metadata in the root
folder. This means updating twice using image_to_live with different images
would break updates.

I've fixed this in the second patch. Please compare P1 vs P2 as P1 is just
the re-landed version of I3695b0903514eb6840e88810b8546fdca690819e. I've
also fixed a bunch of pylint warnings that are now required per upload.

BUG=chromium-os:36990
TEST=Unittests on all changed code, pylint on all changed files, ran update
on x86-generic image using both the serve_only and generate latest workflows.
Ran autoupdate_EndToEndTest

Change-Id: I6bb65b23a34f071e388a4e522fb0fb42eae8781b
Reviewed-on: https://gerrit.chromium.org/gerrit/42271
Tested-by: Chris Sosa <sosa@chromium.org>
Reviewed-by: Gilad Arnold <garnold@chromium.org>
Commit-Queue: Chris Sosa <sosa@chromium.org>
10 files changed