[SPARK-15360][SPARK-SUBMIT] Should print spark-submit usage when no arguments is specified

(Please fill in changes proposed in this fix)
In 2.0, ./bin/spark-submit doesn't print out usage, but it raises an exception.
In this PR, an exception handling is added in the Main.java when the exception is thrown. In the handling code, if there is no additional argument, it prints out usage.

(Please explain how this patch was tested. E.g. unit tests, integration tests, manual tests)
Manually tested.
./bin/spark-submit
Usage: spark-submit [options] <app jar | python file> [app arguments]
Usage: spark-submit --kill [submission ID] --master [spark://...]
Usage: spark-submit --status [submission ID] --master [spark://...]
Usage: spark-submit run-example [options] example-class [example args]

Options:
  --master MASTER_URL         spark://host:port, mesos://host:port, yarn, or local.
  --deploy-mode DEPLOY_MODE   Whether to launch the driver program locally ("client") or
                              on one of the worker machines inside the cluster ("cluster")
                              (Default: client).
  --class CLASS_NAME          Your application's main class (for Java / Scala apps).
  --name NAME                 A name of your application.
  --jars JARS                 Comma-separated list of local jars to include on the driver
                              and executor classpaths.
  --packages                  Comma-separated list of maven coordinates of jars to include
                              on the driver and executor classpaths. Will search the local
                              maven repo, then maven central and any additional remote
                              repositories given by --repositories. The format for the
                              coordinates should be groupId:artifactId:version.

Author: wm624@hotmail.com <wm624@hotmail.com>

Closes #13163 from wangmiao1981/submit.

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

@@ -107,28 +107,37 @@ class SparkSubmitCommandBuilder extends AbstractCommandBuilder {
 
   SparkSubmitCommandBuilder(List<String> args) {
     this.allowsMixedArguments = false;
-
+    this.sparkArgs = new ArrayList<>();
     boolean isExample = false;
     List<String> submitArgs = args;
-    if (args.size() > 0 && args.get(0).equals(PYSPARK_SHELL)) {
-      this.allowsMixedArguments = true;
-      appResource = PYSPARK_SHELL;
-      submitArgs = args.subList(1, args.size());
-    } else if (args.size() > 0 && args.get(0).equals(SPARKR_SHELL)) {
-      this.allowsMixedArguments = true;
-      appResource = SPARKR_SHELL;
-      submitArgs = args.subList(1, args.size());
-    } else if (args.size() > 0 && args.get(0).equals(RUN_EXAMPLE)) {
-      isExample = true;
-      submitArgs = args.subList(1, args.size());
-    }
 
-    this.sparkArgs = new ArrayList<>();
-    this.isExample = isExample;
+    if (args.size() > 0) {
+      switch (args.get(0)) {
+        case PYSPARK_SHELL:
+          this.allowsMixedArguments = true;
+          appResource = PYSPARK_SHELL;
+          submitArgs = args.subList(1, args.size());
+          break;
+
+        case SPARKR_SHELL:
+          this.allowsMixedArguments = true;
+          appResource = SPARKR_SHELL;
+          submitArgs = args.subList(1, args.size());
+          break;
+
+        case RUN_EXAMPLE:
+          isExample = true;
+          submitArgs = args.subList(1, args.size());
+      }
 
-    OptionParser parser = new OptionParser();
-    parser.parse(submitArgs);
-    this.printInfo = parser.infoRequested;
+      this.isExample = isExample;
+      OptionParser parser = new OptionParser();
+      parser.parse(submitArgs);
+      this.printInfo = parser.infoRequested;
+    }  else {
+      this.isExample = isExample;
+      this.printInfo = true;
+    }
   }
 
   @Override
@@ -147,7 +156,7 @@ class SparkSubmitCommandBuilder extends AbstractCommandBuilder {
     List<String> args = new ArrayList<>();
     SparkSubmitOptionParser parser = new SparkSubmitOptionParser();
 
-    if (!allowsMixedArguments) {
+    if (!allowsMixedArguments && !printInfo) {
       checkArgument(appResource != null, "Missing application resource.");
     }
 

launcher/src/test/java/org/apache/spark/launcher/SparkSubmitCommandBuilderSuite.java

@@ -58,6 +58,19 @@ public class SparkSubmitCommandBuilderSuite extends BaseSuite {
     testCmdBuilder(false, false);
   }
 
+  @Test
+  public void testCliHelpAndNoArg() throws Exception {
+    List<String> helpArgs = Arrays.asList(parser.HELP);
+    Map<String, String> env = new HashMap<>();
+    List<String> cmd = buildCommand(helpArgs, env);
+    assertTrue("--help should be contained in the final cmd.", cmd.contains(parser.HELP));
+
+    List<String> sparkEmptyArgs = Collections.emptyList();
+    cmd = buildCommand(sparkEmptyArgs, env);
+    assertTrue("org.apache.spark.deploy.SparkSubmit should be contained in the final cmd of empty input.",
+      cmd.contains("org.apache.spark.deploy.SparkSubmit"));
+  }
+
   @Test
   public void testCliParser() throws Exception {
     List<String> sparkSubmitArgs = Arrays.asList(