Reschedule blocked test jobs
If the slave cannot be contacted for a certain amount of time, undo the assignment of the device and let the job be assigned to a different device.
This may need the allowed_devices support so that the scheduler can know that there is an alternative device.
Note: the first jobs to be reserved for such devices will block other jobs submitted afterwards, so to prevent a cycle starting, this support needs to take the device offline - or be replaced by a health check.
Migrated from https://projects.linaro.org/browse/LAVA-94