[Minor] Fix the value represented by spark.executor.id for consistency.

The property  `spark.executor.id` can represent both `driver` and `<driver>`  for one driver.
It's inconsistent.

This issue is minor so I didn't file this in JIRA.

Author: Kousuke Saruta <sarutak@oss.nttdata.co.jp>

Closes #3812 from sarutak/fix-driver-identifier and squashes the following commits:

d885498 [Kousuke Saruta] Merge branch 'master' of git://git.apache.org/spark into fix-driver-identifier
4275663 [Kousuke Saruta] Fixed the value represented by spark.executor.id of local mode

core/src/main/scala/org/apache/spark/SparkContext.scala

@@ -229,7 +229,7 @@ class SparkContext(config: SparkConf) extends Logging with ExecutorAllocationCli
   // An asynchronous listener bus for Spark events
   private[spark] val listenerBus = new LiveListenerBus
 
-  conf.set("spark.executor.id", "driver")
+  conf.set("spark.executor.id", SparkContext.DRIVER_IDENTIFIER)
 
   // Create the Spark execution environment (cache, map output tracker, etc)
   private[spark] val env = SparkEnv.createDriverEnv(conf, isLocal, listenerBus)