[SPARK-8619] [STREAMING] Don't recover keytab and principal configuration within Streaming checkpoint

[Client.scala](https://github.com/apache/spark/blob/master/yarn/src/main/scala/org/apache/spark/deploy/yarn/Client.scala#L786) will change these configurations, so this would cause the problem that the Streaming recover logic can't find the local keytab file(since configuration was changed)
```scala
      sparkConf.set("spark.yarn.keytab", keytabFileName)
      sparkConf.set("spark.yarn.principal", args.principal)
```

Problem described at [Jira](https://issues.apache.org/jira/browse/SPARK-8619)

Author: huangzhaowei <[email protected]>

Closes #7008 from SaintBacchus/SPARK-8619 and squashes the following commits:

d50dbdf [huangzhaowei] Delect one blank space
9b8e92c [huangzhaowei] Fix code style and add a short comment.
0d8f800 [huangzhaowei] Don't recover keytab and principal configuration within Streaming checkpoint.

(cherry picked from commit d16a944)
Signed-off-by: Tathagata Das <[email protected]>

Author: SaintBacchus

streaming/src/main/scala/org/apache/spark/streaming/Checkpoint.scala

@@ -44,11 +44,23 @@ class Checkpoint(@transient ssc: StreamingContext, val checkpointTime: Time)
   val sparkConfPairs = ssc.conf.getAll
 
   def createSparkConf(): SparkConf = {
+
+    // Reload properties for the checkpoint application since user wants to set a reload property
+    // or spark had changed its value and user wants to set it back.
+    val propertiesToReload = List(
+      "spark.master",
+      "spark.yarn.keytab",
+      "spark.yarn.principal")
+
     val newSparkConf = new SparkConf(loadDefaults = false).setAll(sparkConfPairs)
       .remove("spark.driver.host")
       .remove("spark.driver.port")
-    val newMasterOption = new SparkConf(loadDefaults = true).getOption("spark.master")
-    newMasterOption.foreach { newMaster => newSparkConf.setMaster(newMaster) }
+    val newReloadConf = new SparkConf(loadDefaults = true)
+    propertiesToReload.foreach { prop =>
+      newReloadConf.getOption(prop).foreach { value =>
+        newSparkConf.set(prop, value)
+      }
+    }
     newSparkConf
   }