[SPARK-18740] Log spark.app.name in driver logs

## What changes were proposed in this pull request?

Added simple logInfo line to print out the `spark.app.name` in the driver logs

## How was this patch tested?

Spark was built and tested with SparkPi app. Example log:
```
16/12/06 05:49:50 INFO spark.SparkContext: Running Spark version 2.0.0
16/12/06 05:49:52 INFO spark.SparkContext: Submitted application: Spark Pi
16/12/06 05:49:52 INFO spark.SecurityManager: Changing view acls to: root
16/12/06 05:49:52 INFO spark.SecurityManager: Changing modify acls to: root
```

Author: Peter Ableda <peter.ableda@cloudera.com>

Closes #16172 from peterableda/feature/print_appname.

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

@@ -382,6 +382,9 @@ class SparkContext(config: SparkConf) extends Logging {
       throw new SparkException("An application name must be set in your configuration")
     }
 
+    // log out spark.app.name in the Spark driver logs
+    logInfo(s"Submitted application: $appName")
+
     // System property spark.yarn.app.id must be set if user code ran by AM on a YARN cluster
     if (master == "yarn" && deployMode == "cluster" && !_conf.contains("spark.yarn.app.id")) {
       throw new SparkException("Detected yarn cluster mode, but isn't running on a cluster. " +