[jobbrowser] Fix fetching task attempt logs for completed tasks

It appears that we are not properly fetching the task attempt logs when a job has finished and all the logs have been migrated to the job history server. Instead, we're trying to talk to the node that ran the task attempt. It's possible the reason this hasn't been reproduced is that we may be running our tests with the YARN config option yarn.nodemanager.delete.debug-delay-sec set, which keeps those log files around for some period of time.