[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.

4 files changed, 9 insertions, 16 deletions

diff --git a/core/src/test/scala/org/apache/spark/DistributedSuite.scala b/core/src/test/scala/org/apache/spark/DistributedSuite.scala
index 67d722c1dc..2110d3d770 100644
--- a/core/src/test/scala/org/apache/spark/DistributedSuite.scala
+++ b/core/src/test/scala/org/apache/spark/DistributedSuite.scala
@@ -320,7 +320,7 @@ class DistributedSuite extends SparkFunSuite with Matchers with LocalSparkContex
           Thread.sleep(200)
         }
       } catch {
-        case _: Throwable => { Thread.sleep(10) }
+        case _: Throwable => Thread.sleep(10)
           // Do nothing. We might see exceptions because block manager
           // is racing this thread to remove entries from the driver.
       }
diff --git a/core/src/test/scala/org/apache/spark/SparkContextInfoSuite.scala b/core/src/test/scala/org/apache/spark/SparkContextInfoSuite.scala
index 3706455c3f..8feb3dee05 100644
--- a/core/src/test/scala/org/apache/spark/SparkContextInfoSuite.scala
+++ b/core/src/test/scala/org/apache/spark/SparkContextInfoSuite.scala
@@ -82,20 +82,18 @@ package object testPackage extends Assertions {
     val curCallSite = sc.getCallSite().shortForm // note: 2 lines after definition of "rdd"
 
     val rddCreationLine = rddCreationSite match {
-      case CALL_SITE_REGEX(func, file, line) => {
+      case CALL_SITE_REGEX(func, file, line) =>
         assert(func === "makeRDD")
         assert(file === "SparkContextInfoSuite.scala")
         line.toInt
-      }
       case _ => fail("Did not match expected call site format")
     }
 
     curCallSite match {
-      case CALL_SITE_REGEX(func, file, line) => {
+      case CALL_SITE_REGEX(func, file, line) =>
         assert(func === "getCallSite") // this is correct because we called it from outside of Spark
         assert(file === "SparkContextInfoSuite.scala")
         assert(line.toInt === rddCreationLine.toInt + 2)
-      }
       case _ => fail("Did not match expected call site format")
     }
   }
diff --git a/core/src/test/scala/org/apache/spark/UnpersistSuite.scala b/core/src/test/scala/org/apache/spark/UnpersistSuite.scala
index f7a13ab399..09e21646ee 100644
--- a/core/src/test/scala/org/apache/spark/UnpersistSuite.scala
+++ b/core/src/test/scala/org/apache/spark/UnpersistSuite.scala
@@ -35,7 +35,7 @@ class UnpersistSuite extends SparkFunSuite with LocalSparkContext {
           Thread.sleep(200)
         }
       } catch {
-        case _: Throwable => { Thread.sleep(10) }
+        case _: Throwable => Thread.sleep(10)
           // Do nothing. We might see exceptions because block manager
           // is racing this thread to remove entries from the driver.
       }
diff --git a/core/src/test/scala/org/apache/spark/rpc/RpcEnvSuite.scala b/core/src/test/scala/org/apache/spark/rpc/RpcEnvSuite.scala
index 43e61241b6..cebac2097f 100644
--- a/core/src/test/scala/org/apache/spark/rpc/RpcEnvSuite.scala
+++ b/core/src/test/scala/org/apache/spark/rpc/RpcEnvSuite.scala
@@ -127,9 +127,8 @@ abstract class RpcEnvSuite extends SparkFunSuite with BeforeAndAfterAll {
       override val rpcEnv = env
 
       override def receiveAndReply(context: RpcCallContext): PartialFunction[Any, Unit] = {
-        case msg: String => {
+        case msg: String =>
           context.reply(msg)
-        }
       }
     })
     val reply = rpcEndpointRef.askWithRetry[String]("hello")
@@ -141,9 +140,8 @@ abstract class RpcEnvSuite extends SparkFunSuite with BeforeAndAfterAll {
       override val rpcEnv = env
 
       override def receiveAndReply(context: RpcCallContext): PartialFunction[Any, Unit] = {
-        case msg: String => {
+        case msg: String =>
           context.reply(msg)
-        }
       }
     })
 
@@ -164,10 +162,9 @@ abstract class RpcEnvSuite extends SparkFunSuite with BeforeAndAfterAll {
       override val rpcEnv = env
 
       override def receiveAndReply(context: RpcCallContext): PartialFunction[Any, Unit] = {
-        case msg: String => {
+        case msg: String =>
           Thread.sleep(100)
           context.reply(msg)
-        }
       }
     })
 
@@ -317,10 +314,9 @@ abstract class RpcEnvSuite extends SparkFunSuite with BeforeAndAfterAll {
       override val rpcEnv = env
 
       override def receive: PartialFunction[Any, Unit] = {
-        case m => {
+        case m =>
           self
           callSelfSuccessfully = true
-        }
       }
     })
 
@@ -682,9 +678,8 @@ abstract class RpcEnvSuite extends SparkFunSuite with BeforeAndAfterAll {
         override val rpcEnv = localEnv
 
         override def receiveAndReply(context: RpcCallContext): PartialFunction[Any, Unit] = {
-          case msg: String => {
+          case msg: String =>
             context.reply(msg)
-          }
         }
       })
       val rpcEndpointRef = remoteEnv.setupEndpointRef(localEnv.address, "ask-authentication")