supervisord config: tidy syntax, consolidate logs

- Consistently use spaces around 'key = val' equal sign to make them
  visually distinct from command-line args.
- Combine stdout & stderr in one log; specify log file location.
- Document logging behavior.

Change-Id: I62a80db6ab962d47074f5246f97641bdb04369e7
Merge "supervisord config: tidy syntax, consolidate logs"