[SPARK-20521][DOC][CORE] The default of 'spark.worker.cleanup.appDataTtl' should be 604800 in spark-standalone.md

## What changes were proposed in this pull request?

Currently, our project needs to be set to clean up the worker directory cleanup cycle is three days.
When I follow http://spark.apache.org/docs/latest/spark-standalone.html, configure the 'spark.worker.cleanup.appDataTtl' parameter, I configured to 3 * 24 * 3600.
When I start the spark service, the startup fails, and the worker log displays the error log as follows:

2017-04-28 15:02:03,306 INFO Utils: Successfully started service 'sparkWorker' on port 48728.
Exception in thread "main" java.lang.NumberFormatException: For input string: "3 * 24 * 3600"
	at java.lang.NumberFormatException.forInputString(NumberFormatException.java:65)
	at java.lang.Long.parseLong(Long.java:430)
	at java.lang.Long.parseLong(Long.java:483)
	at scala.collection.immutable.StringLike$class.toLong(StringLike.scala:276)
	at scala.collection.immutable.StringOps.toLong(StringOps.scala:29)
	at org.apache.spark.SparkConf$$anonfun$getLong$2.apply(SparkConf.scala:380)
	at org.apache.spark.SparkConf$$anonfun$getLong$2.apply(SparkConf.scala:380)
	at scala.Option.map(Option.scala:146)
	at org.apache.spark.SparkConf.getLong(SparkConf.scala:380)
	at org.apache.spark.deploy.worker.Worker.<init>(Worker.scala:100)
	at org.apache.spark.deploy.worker.Worker$.startRpcEnvAndEndpoint(Worker.scala:730)
	at org.apache.spark.deploy.worker.Worker$.main(Worker.scala:709)
	at org.apache.spark.deploy.worker.Worker.main(Worker.scala)

**Because we put 7 * 24 * 3600 as a string, forced to convert to the dragon type,  will lead to problems in the program.**

**So I think the default value of the current configuration should be a specific long value, rather than 7 * 24 * 3600,should be 604800. Because it would mislead users for similar configurations, resulting in spark start failure.**

## How was this patch tested?
manual tests

Please review http://spark.apache.org/contributing.html before opening a pull request.

Author: 郭小龙 10207633 <guo.xiaolong1@zte.com.cn>
Author: guoxiaolong <guo.xiaolong1@zte.com.cn>
Author: guoxiaolongzte <guo.xiaolong1@zte.com.cn>

Closes #17798 from guoxiaolongzte/SPARK-20521.