SPARK-2083 Add support for spark.local.maxFailures configuration property

The logic in `SparkContext` for creating a new task scheduler now looks for a "spark.local.maxFailures" property to specify the number of task failures in a local job that will cause the job to fail. Its default is the prior fixed value of 1 (no retries).

The patch includes documentation updates and new unit tests.
i'd rename the variable maxTaskFailures