[SPARK-8650] [SQL] Use the user-specified app name priority in SparkSQLCLIDriver or HiveThriftServer2

When run `./bin/spark-sql --name query1.sql`
[Before]
![before](https://cloud.githubusercontent.com/assets/1400819/8370336/fa20b75a-1bf8-11e5-9171-040049a53240.png)

[After]
![after](https://cloud.githubusercontent.com/assets/1400819/8370189/dcc35cb4-1bf6-11e5-8796-a0694140bffb.png)

Author: Yadong Qi <qiyadong2010@gmail.com>

Closes apache#7030 from watermen/SPARK-8650 and squashes the following commits:

51b5134 [Yadong Qi] Improve code and add comment.
e3d7647 [Yadong Qi] use spark.app.name priority.

Author: watermen

sql/hive-thriftserver/src/main/scala/org/apache/spark/sql/hive/thriftserver/SparkSQLEnv.scala

@@ -38,9 +38,14 @@ private[hive] object SparkSQLEnv extends Logging {
       val sparkConf = new SparkConf(loadDefaults = true)
       val maybeSerializer = sparkConf.getOption("spark.serializer")
       val maybeKryoReferenceTracking = sparkConf.getOption("spark.kryo.referenceTracking")
+      // If user doesn't specify the appName, we want to get [SparkSQL::localHostName] instead of
+      // the default appName [SparkSQLCLIDriver] in cli or beeline.
+      val maybeAppName = sparkConf
+        .getOption("spark.app.name")
+        .filterNot(_ == classOf[SparkSQLCLIDriver].getName)
 
       sparkConf
-        .setAppName(s"SparkSQL::${Utils.localHostName()}")
+        .setAppName(maybeAppName.getOrElse(s"SparkSQL::${Utils.localHostName()}"))
         .set(
           "spark.serializer",
           maybeSerializer.getOrElse("org.apache.spark.serializer.KryoSerializer"))