SPARK-1350. Always use JAVA_HOME to run executor container JVMs.

Author: Sandy Ryza <sandy@cloudera.com>

Closes #313 from sryza/sandy-spark-1350 and squashes the following commits:

bb6d187 [Sandy Ryza] SPARK-1350. Always use JAVA_HOME to run executor container JVMs.

yarn/common/src/main/scala/org/apache/spark/deploy/yarn/ClientBase.scala

@@ -345,14 +345,8 @@ trait ClientBase extends Logging {
     }
 
     // Command for the ApplicationMaster
-    var javaCommand = "java"
-    val javaHome = System.getenv("JAVA_HOME")
-    if ((javaHome != null && !javaHome.isEmpty()) || env.isDefinedAt("JAVA_HOME")) {
-      javaCommand = Environment.JAVA_HOME.$() + "/bin/java"
-    }
-
     val commands = List[String](
-      javaCommand +
+      Environment.JAVA_HOME.$() + "/bin/java" +
         " -server " +
         JAVA_OPTS +
         " " + args.amClass +

yarn/common/src/main/scala/org/apache/spark/deploy/yarn/ExecutorRunnableUtil.scala

@@ -88,13 +88,8 @@ trait ExecutorRunnableUtil extends Logging {
         }
     */
 
-    var javaCommand = "java"
-    val javaHome = System.getenv("JAVA_HOME")
-    if ((javaHome != null && !javaHome.isEmpty()) || env.isDefinedAt("JAVA_HOME")) {
-      javaCommand = Environment.JAVA_HOME.$() + "/bin/java"
-    }
-
-    val commands = List[String](javaCommand +
+    val commands = List[String](
+      Environment.JAVA_HOME.$() + "/bin/java" +
       " -server " +
       // Kill if OOM is raised - leverage yarn's failure handling to cause rescheduling.
       // Not killing the task leaves various aspects of the executor and (to some extent) the jvm in