paygen: Re-enable generation of NMO update test payloads.

This CL adds more cataloging of NMO/previous builds during
the build discovery phase of paygen.

This information is used to better control which test payloads
we generate.

This CL removes any usage of '_previous_versions' in paygen and allows
for all discovered 'previous' builds to have update test payloads
generated.

BUG=chromium:541417
TEST=Unittests.

Reviewed-on: https://chromium-review.googlesource.com/305332
Commit-Ready: Matthew Sartori <msartori@chromium.org>
Tested-by: Matthew Sartori <msartori@chromium.org>
Reviewed-by: Don Garrett <dgarrett@chromium.org>
(cherry picked from commit 3ecd1eb51da8cf67bc987b90b1d8df0f8f40334f)

Change-Id: Ie927de458dde6cebda6eb03051d2f85b4e8d2286
Reviewed-on: https://chromium-review.googlesource.com/309569
Reviewed-by: Josafat Garcia <josafat@chromium.org>
Tested-by: Matthew Sartori <msartori@chromium.org>
2 files changed