Gracefully handle two parallel DAGs #445

Point is to handle the case where to DAGs are running and they share the
same user log.

Test suite is not able to find default log.
Fix job_dagman_recovery_event_check test.

This prepends the schedd host and (DAGMan) cluster id to the default
node log, so that the different running DAGMan jobs will not see the
same events.  But I  need to think more carefully how this affects
recovery.