[Import Notifier] Get Monorail OAuth token from luci-auth

The application default credential belongs to the default Swarming bot
account, not the dedicated service account for wpt-importer. As
suggested by LUCI task force, we need to use `luci-auth token` instead.

Bug: 790503
Change-Id: I3257db9a49196fd427c6f2371d00dcabab7ee3a6
Reviewed-on: https://chromium-review.googlesource.com/c/1315931
Commit-Queue: Robert Ma <robertma@chromium.org>
Reviewed-by: Quinten Yearsley <qyearsley@chromium.org>
Cr-Commit-Position: refs/heads/master@{#605746}
4 files changed