[SPARK-14508][BUILD] Add a new ScalaStyle Rule `OmitBracesInCase`

## What changes were proposed in this pull request?

According to the [Spark Code Style Guide](https://cwiki.apache.org/confluence/display/SPARK/Spark+Code+Style+Guide) and [Scala Style Guide](http://docs.scala-lang.org/style/control-structures.html#curlybraces), we had better enforce the following rule.
  ```
  case: Always omit braces in case clauses.
  ```
This PR makes a new ScalaStyle rule, 'OmitBracesInCase', and enforces it to the code.

## How was this patch tested?

Pass the Jenkins tests (including Scala style checking)

Author: Dongjoon Hyun <dongjoon@apache.org>

Closes #12280 from dongjoon-hyun/SPARK-14508.

1 files changed, 8 insertions, 16 deletions

diff --git a/yarn/src/test/scala/org/apache/spark/deploy/yarn/YarnSparkHadoopUtilSuite.scala b/yarn/src/test/scala/org/apache/spark/deploy/yarn/YarnSparkHadoopUtilSuite.scala
index de14e36f4e..fe09808ae5 100644
--- a/yarn/src/test/scala/org/apache/spark/deploy/yarn/YarnSparkHadoopUtilSuite.scala
+++ b/yarn/src/test/scala/org/apache/spark/deploy/yarn/YarnSparkHadoopUtilSuite.scala
@@ -101,22 +101,18 @@ class YarnSparkHadoopUtilSuite extends SparkFunSuite with Matchers with Logging
     val modifyAcls = acls.get(ApplicationAccessType.MODIFY_APP)
 
     viewAcls match {
-      case Some(vacls) => {
+      case Some(vacls) =>
         val aclSet = vacls.split(',').map(_.trim).toSet
         assert(aclSet.contains(System.getProperty("user.name", "invalid")))
-      }
-      case None => {
+      case None =>
         fail()
-      }
     }
     modifyAcls match {
-      case Some(macls) => {
+      case Some(macls) =>
         val aclSet = macls.split(',').map(_.trim).toSet
         assert(aclSet.contains(System.getProperty("user.name", "invalid")))
-      }
-      case None => {
+      case None =>
         fail()
-      }
     }
   }
 
@@ -135,26 +131,22 @@ class YarnSparkHadoopUtilSuite extends SparkFunSuite with Matchers with Logging
     val modifyAcls = acls.get(ApplicationAccessType.MODIFY_APP)
 
     viewAcls match {
-      case Some(vacls) => {
+      case Some(vacls) =>
         val aclSet = vacls.split(',').map(_.trim).toSet
         assert(aclSet.contains("user1"))
         assert(aclSet.contains("user2"))
         assert(aclSet.contains(System.getProperty("user.name", "invalid")))
-      }
-      case None => {
+      case None =>
         fail()
-      }
     }
     modifyAcls match {
-      case Some(macls) => {
+      case Some(macls) =>
         val aclSet = macls.split(',').map(_.trim).toSet
         assert(aclSet.contains("user3"))
         assert(aclSet.contains("user4"))
         assert(aclSet.contains(System.getProperty("user.name", "invalid")))
-      }
-      case None => {
+      case None =>
         fail()
-      }
     }
 
   }