[SPARK-7261] [CORE] Change default log level to WARN in the REPL

1. Add `log4j-defaults-repl.properties` that has log level WARN.
2. When logging is initialized, check whether inside the REPL. If so, use `log4j-defaults-repl.properties`.
3. Print the following information if using `log4j-defaults-repl.properties`:
```
Using Spark's repl log4j profile: org/apache/spark/log4j-defaults-repl.properties
To adjust logging level use sc.setLogLevel("INFO")
```

Author: zsxwing <zsxwing@gmail.com>

Closes #6734 from zsxwing/log4j-repl and squashes the following commits:

3835eff [zsxwing] Change default log level to WARN in the REPL

.rat-excludes

@@ -28,6 +28,7 @@ spark-env.sh
 spark-env.cmd
 spark-env.sh.template
 log4j-defaults.properties
+log4j-defaults-repl.properties
 bootstrap-tooltip.js
 jquery-1.11.1.min.js
 d3.min.js

core/src/main/resources/org/apache/spark/log4j-defaults-repl.properties

+# Set everything to be logged to the console
+log4j.rootCategory=WARN, console
+log4j.appender.console=org.apache.log4j.ConsoleAppender
+log4j.appender.console.target=System.err
+log4j.appender.console.layout=org.apache.log4j.PatternLayout
+log4j.appender.console.layout.ConversionPattern=%d{yy/MM/dd HH:mm:ss} %p %c{1}: %m%n
+
+# Settings to quiet third party logs that are too verbose
+log4j.logger.org.spark-project.jetty=WARN
+log4j.logger.org.spark-project.jetty.util.component.AbstractLifeCycle=ERROR
+log4j.logger.org.apache.spark.repl.SparkIMain$exprTyper=INFO
+log4j.logger.org.apache.spark.repl.SparkILoop$SparkILoopInterpreter=INFO

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

@@ -121,13 +121,25 @@ trait Logging {
     if (usingLog4j12) {
       val log4j12Initialized = LogManager.getRootLogger.getAllAppenders.hasMoreElements
       if (!log4j12Initialized) {
-        val defaultLogProps = "org/apache/spark/log4j-defaults.properties"
-        Option(Utils.getSparkClassLoader.getResource(defaultLogProps)) match {
-          case Some(url) =>
-            PropertyConfigurator.configure(url)
-            System.err.println(s"Using Spark's default log4j profile: $defaultLogProps")
-          case None =>
-            System.err.println(s"Spark was unable to load $defaultLogProps")
+        if (Utils.isInInterpreter) {
+          val replDefaultLogProps = "org/apache/spark/log4j-defaults-repl.properties"
+          Option(Utils.getSparkClassLoader.getResource(replDefaultLogProps)) match {
+            case Some(url) =>
+              PropertyConfigurator.configure(url)
+              System.err.println(s"Using Spark's repl log4j profile: $replDefaultLogProps")
+              System.err.println("To adjust logging level use sc.setLogLevel(\"INFO\")")
+            case None =>
+              System.err.println(s"Spark was unable to load $replDefaultLogProps")
+          }
+        } else {
+          val defaultLogProps = "org/apache/spark/log4j-defaults.properties"
+          Option(Utils.getSparkClassLoader.getResource(defaultLogProps)) match {
+            case Some(url) =>
+              PropertyConfigurator.configure(url)
+              System.err.println(s"Using Spark's default log4j profile: $defaultLogProps")
+            case None =>
+              System.err.println(s"Spark was unable to load $defaultLogProps")
+          }
         }
       }
     }