[SPARK-4881][Minor] Use SparkConf#getBoolean instead of get().toBoolean

It's really a minor issue.

In ApplicationMaster, there is code like as follows.

    val preserveFiles = sparkConf.get("spark.yarn.preserve.staging.files", "false").toBoolean


I think, the code can be simplified like as follows.

    val preserveFiles = sparkConf.getBoolean("spark.yarn.preserve.staging.files", false)