[autotest] Don't automatically create shards in heartbeats

Shards are added and assigned to labels using atest. If they
already exist this can not be done. Therefore they must not
automatically be created on their first heartbeat. Also it makes
more sense to fail instead of silently accepting unknown heartbeats.

BUG=None
DEPLOY=apache
TEST=Ran suites.

Change-Id: Id485ad956c83c16dc96916ba58e25c6f91028dd8
Reviewed-on: https://chromium-review.googlesource.com/219423
Reviewed-by: Alex Miller <milleral@chromium.org>
Commit-Queue: Jakob Jülich <jakobjuelich@chromium.org>
Tested-by: Jakob Jülich <jakobjuelich@chromium.org>
2 files changed