To run REEF as a library, we need to be able to co-locate REEF Driver with some other Application Master process (e.g. Spark AM or other REEF Driver). To that end, we need to introduce a driver configuration flag (e.g. DriverIsUnmanaged), and allow REEF runtime to check it and set the proper value in AM request (e.g. ApplicationSubmissionContext.setUnmanagedAM() in YARN API).