[autotest] In test_push, try all devservers to locate a stumpy build or use stable build

BUG=chromium:370089
TEST=run test_push locally, unittest

Change-Id: Ifa2d18a48f9eed1f2af47e7a2989a7cd5ad82480
Reviewed-on: https://chromium-review.googlesource.com/199144
Reviewed-by: Dan Shi <dshi@chromium.org>
Commit-Queue: Dan Shi <dshi@chromium.org>
Tested-by: Dan Shi <dshi@chromium.org>
2 files changed