Find a way to easily arrange logs of multiple dolphin jobs in a single REEF job

[wynot12]

In our job server implementation, logs from different jobs go into a single log file, without a way to know which job the log is about.

We need to provide a way to classify them. Possible approaches are:

• store each job's log into a separate file
• append job id field in all of job-specific logs

[wynot12]

I'll send a PR for this when #1264 is merged.