Example: 

https://ubercautotest.corp.google.com/afe/#tab_id=view_job&object_id=215834749 cannot be scheduled.

It cannot be picked up by the shard due to:
1) it doesn't have any job dependency label like 'board:lulu', OR
2) the DUT chromeos15-row3-rack1-host2 cannot be recognized in shard heartbeat.

For 1), in job creating via afe interface, there's no way to pass in a job dependency label.

For 2), it's because the newly added DUT chromeos15-row3-rack1-host2 (Not sure whether there're others) doesn't have correct static labels stored in afe_static_host_labels.

The server cautotest-prod has 'respect_static_label=False', which means even a DUT is added by 'atest host create', it won't be assigned any static labels.

However, when a job is scheduled to cautotest-prod, in shardheartbeat, it uses static_label to get the job lists that assigned to a shard:
https://cs.corp.google.com/chromeos_public/src/third_party/autotest/files/frontend/afe/models.py?q=classify_label_objects&sq=package:%5Echromeos_(internal%7Cpublic)$&g=0&l=1696

Which makes the above job 215834749 cannot be picked up.

First, let's make shard-heartbeat also respect "respect_static_label" when getting job list.