[SPARK-26998][CORE] Remove SSL configuration from executors

## What changes were proposed in this pull request?

Different SSL passwords shown up as command line argument on executor side in standalone mode:
* keyStorePassword
* keyPassword
* trustStorePassword

In this PR I've removed SSL configurations from executors.

## How was this patch tested?

Existing + additional unit tests.
Additionally tested with standalone mode and checked the command line arguments:
```
[gaborsomogyi:~/spark] SPARK-26998(+4/-0,3)+ ± jps
94803 CoarseGrainedExecutorBackend
94818 Jps
90149 RemoteMavenServer
91925 Nailgun
94793 SparkSubmit
94680 Worker
94556 Master
398
[gaborsomogyi:~/spark] SPARK-26998(+4/-1,3)+ ± ps -ef | egrep "94556|94680|94793|94803"
  502 94556     1   0  2:02PM ttys007    0:07.39 /Library/Java/JavaVirtualMachines/jdk1.8.0_152.jdk/Contents/Home/bin/java -cp /Users/gaborsomogyi/spark/conf/:/Users/gaborsomogyi/spark/assembly/target/scala-2.12/jars/* -Xmx1g org.apache.spark.deploy.master.Master --host gsomogyi-MBP.local --port 7077 --webui-port 8080 --properties-file conf/spark-defaults.conf
  502 94680     1   0  2:02PM ttys007    0:07.27 /Library/Java/JavaVirtualMachines/jdk1.8.0_152.jdk/Contents/Home/bin/java -cp /Users/gaborsomogyi/spark/conf/:/Users/gaborsomogyi/spark/assembly/target/scala-2.12/jars/* -Xmx1g org.apache.spark.deploy.worker.Worker --webui-port 8081 --properties-file conf/spark-defaults.conf spark://gsomogyi-MBP.local:7077
  502 94793 94782   0  2:02PM ttys007    0:35.52 /Library/Java/JavaVirtualMachines/jdk1.8.0_152.jdk/Contents/Home/bin/java -cp /Users/gaborsomogyi/spark/conf/:/Users/gaborsomogyi/spark/assembly/target/scala-2.12/jars/* -Dscala.usejavacp=true -Xmx1g org.apache.spark.deploy.SparkSubmit --master spark://gsomogyi-MBP.local:7077 --class org.apache.spark.repl.Main --name Spark shell spark-shell
  502 94803 94680   0  2:03PM ttys007    0:05.20 /Library/Java/JavaVirtualMachines/jdk1.8.0_152.jdk/Contents/Home/bin/java -cp /Users/gaborsomogyi/spark/conf/:/Users/gaborsomogyi/spark/assembly/target/scala-2.12/jars/* -Xmx1024M -Dspark.ssl.ui.port=0 -Dspark.driver.port=60902 org.apache.spark.executor.CoarseGrainedExecutorBackend --driver-url spark://CoarseGrainedScheduler172.30.65.186:60902 --executor-id 0 --hostname 172.30.65.186 --cores 8 --app-id app-20190326140311-0000 --worker-url spark://Worker172.30.65.186:60899
  502 94910 57352   0  2:05PM ttys008    0:00.00 egrep 94556|94680|94793|94803
```

Closes apache#24170 from gaborgsomogyi/SPARK-26998.

Authored-by: Gabor Somogyi <[email protected]>
Signed-off-by: Marcelo Vanzin <[email protected]>

Author: gaborgsomogyi

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

@@ -738,7 +738,6 @@ private[spark] object SparkConf extends Logging {
    */
   def isExecutorStartupConf(name: String): Boolean = {
     (name.startsWith("spark.auth") && name != SecurityManager.SPARK_AUTH_SECRET_CONF) ||
-    name.startsWith("spark.ssl") ||
     name.startsWith("spark.rpc") ||
     name.startsWith("spark.network") ||
     isSparkPortConf(name)

core/src/test/scala/org/apache/spark/SparkConfSuite.scala

@@ -353,6 +353,17 @@ class SparkConfSuite extends SparkFunSuite with LocalSparkContext with ResetSyst
     }
   }
 
+  test("SPARK-26998: SSL configuration not needed on executors") {
+    val conf = new SparkConf(false)
+    conf.set("spark.ssl.enabled", "true")
+    conf.set("spark.ssl.keyPassword", "password")
+    conf.set("spark.ssl.keyStorePassword", "password")
+    conf.set("spark.ssl.trustStorePassword", "password")
+
+    val filtered = conf.getAll.filter { case (k, _) => SparkConf.isExecutorStartupConf(k) }
+    assert(filtered.isEmpty)
+  }
+
   test("SPARK-27244 toDebugString redacts sensitive information") {
     val conf = new SparkConf(loadDefaults = false)
       .set("dummy.password", "dummy-password")