Make it configurable to retrieve reducer task reports in JobWarnings

In Hadoop 2, the job history server (JHS) is not that stable. The problem is that any call to JHS can make Pig jobs get stuck if JHS is not responding. This is often an issue in production ETL flow.

So Lipstick should skip calling [Map|Reduce]TaskReports() when "pig.stats.notaskreport" is set to true.