[SPARK-20814][MESOS] Restore support for spark.executor.extraClassPath.

Restore code that was removed as part of SPARK-17979, but instead of
using the deprecated env variable name to propagate the class path, use
a new one.

Verified by running "./bin/spark-class o.a.s.executor.CoarseGrainedExecutorBackend"
manually.

Author: Marcelo Vanzin <vanzin@cloudera.com>

Closes #18037 from vanzin/SPARK-20814.

launcher/src/main/java/org/apache/spark/launcher/SparkClassCommandBuilder.java

@@ -68,10 +68,12 @@ class SparkClassCommandBuilder extends AbstractCommandBuilder {
       case "org.apache.spark.executor.CoarseGrainedExecutorBackend":
         javaOptsKeys.add("SPARK_EXECUTOR_OPTS");
         memKey = "SPARK_EXECUTOR_MEMORY";
+        extraClassPath = getenv("SPARK_EXECUTOR_CLASSPATH");
         break;
       case "org.apache.spark.executor.MesosExecutorBackend":
         javaOptsKeys.add("SPARK_EXECUTOR_OPTS");
         memKey = "SPARK_EXECUTOR_MEMORY";
+        extraClassPath = getenv("SPARK_EXECUTOR_CLASSPATH");
         break;
       case "org.apache.spark.deploy.mesos.MesosClusterDispatcher":
         javaOptsKeys.add("SPARK_DAEMON_JAVA_OPTS");

resource-managers/mesos/src/main/scala/org/apache/spark/scheduler/cluster/mesos/MesosCoarseGrainedSchedulerBackend.scala

@@ -185,6 +185,11 @@ private[spark] class MesosCoarseGrainedSchedulerBackend(
 
   def createCommand(offer: Offer, numCores: Int, taskId: String): CommandInfo = {
     val environment = Environment.newBuilder()
+    val extraClassPath = conf.getOption("spark.executor.extraClassPath")
+    extraClassPath.foreach { cp =>
+      environment.addVariables(
+        Environment.Variable.newBuilder().setName("SPARK_EXECUTOR_CLASSPATH").setValue(cp).build())
+    }
     val extraJavaOpts = conf.get("spark.executor.extraJavaOptions", "")
 
     // Set the environment variable through a command prefix

resource-managers/mesos/src/main/scala/org/apache/spark/scheduler/cluster/mesos/MesosFineGrainedSchedulerBackend.scala

@@ -106,6 +106,10 @@ private[spark] class MesosFineGrainedSchedulerBackend(
       throw new SparkException("Executor Spark home `spark.mesos.executor.home` is not set!")
     }
     val environment = Environment.newBuilder()
+    sc.conf.getOption("spark.executor.extraClassPath").foreach { cp =>
+      environment.addVariables(
+        Environment.Variable.newBuilder().setName("SPARK_EXECUTOR_CLASSPATH").setValue(cp).build())
+    }
     val extraJavaOpts = sc.conf.getOption("spark.executor.extraJavaOptions").getOrElse("")
 
     val prefixEnv = sc.conf.getOption("spark.executor.extraLibraryPath").map { p =>