[SPARK-5559] [Streaming] [Test] Remove oppotunity we met flakiness when running FlumeStreamSuite

When we run FlumeStreamSuite on Jenkins, sometimes we get error like as follows.

    sbt.ForkMain$ForkError: The code passed to eventually never returned normally. Attempted 52 times over 10.094849836 seconds. Last failure message: Error connecting to localhost/127.0.0.1:23456.
	    at org.scalatest.concurrent.Eventually$class.tryTryAgain$1(Eventually.scala:420)
	    at org.scalatest.concurrent.Eventually$class.eventually(Eventually.scala:438)
	    at org.scalatest.concurrent.Eventually$.eventually(Eventually.scala:478)
	    at org.scalatest.concurrent.Eventually$class.eventually(Eventually.scala:307)
	   at org.scalatest.concurrent.Eventually$.eventually(Eventually.scala:478)
	   at org.apache.spark.streaming.flume.FlumeStreamSuite.writeAndVerify(FlumeStreamSuite.scala:116)
           at org.apache.spark.streaming.flume.FlumeStreamSuite.org$apache$spark$streaming$flume$FlumeStreamSuite$$testFlumeStream(FlumeStreamSuite.scala:74)
	   at org.apache.spark.streaming.flume.FlumeStreamSuite$$anonfun$3.apply$mcV$sp(FlumeStreamSuite.scala:66)
	    at org.apache.spark.streaming.flume.FlumeStreamSuite$$anonfun$3.apply(FlumeStreamSuite.scala:66)
	    at org.apache.spark.streaming.flume.FlumeStreamSuite$$anonfun$3.apply(FlumeStreamSuite.scala:66)
	    at org.scalatest.Transformer$$anonfun$apply$1.apply$mcV$sp(Transformer.scala:22)
	    at org.scalatest.OutcomeOf$class.outcomeOf(OutcomeOf.scala:85)
	    at org.scalatest.OutcomeOf$.outcomeOf(OutcomeOf.scala:104)
	    at org.scalatest.Transformer.apply(Transformer.scala:22)
	    at org.scalatest.Transformer.apply(Transformer.scala:20)
    	    at org.scalatest.FunSuiteLike$$anon$1.apply(FunSuiteLike.scala:166)
	    at org.scalatest.Suite$class.withFixture(Suite.scala:1122)
	    at org.scalatest.FunSuite.withFixture(FunSuite.scala:1555)
	    at org.scalatest.FunSuiteLike$class.invokeWithFixture$1(FunSuiteLike.scala:163)
	   at org.scalatest.FunSuiteLike$$anonfun$runTest$1.apply(FunSuiteLike.scala:175)
	    at org.scalatest.FunSuiteLike$$anonfun$runTest$1.apply(FunSuiteLike.scala:175)
	    at org.scalatest.SuperEngine.runTestImpl(Engine.scala:306)
	    at org.scalatest.FunSuiteLike$class.runTest(FunSuiteLike.scala:175)

This error is caused by check-then-act logic  when it find free-port .

      /** Find a free port */
      private def findFreePort(): Int = {
        Utils.startServiceOnPort(23456, (trialPort: Int) => {
          val socket = new ServerSocket(trialPort)
          socket.close()
          (null, trialPort)
        }, conf)._2
      }

Removing the check-then-act is not easy but we can reduce the chance of having the error by choosing random value for initial port instead of 23456.

Author: Kousuke Saruta <sarutak@oss.nttdata.co.jp>

Closes #4337 from sarutak/SPARK-5559 and squashes the following commits:

16f109f [Kousuke Saruta] Added `require` to Utils#startServiceOnPort
c39d8b6 [Kousuke Saruta] Merge branch 'SPARK-5559' of github.com:sarutak/spark into SPARK-5559
1610ba2 [Kousuke Saruta] Merge branch 'master' of git://git.apache.org/spark into SPARK-5559
33357e3 [Kousuke Saruta] Changed "findFreePort" method in MQTTStreamSuite and FlumeStreamSuite so that it can choose valid random port
a9029fe [Kousuke Saruta] Merge branch 'master' of git://git.apache.org/spark into SPARK-5559
9489ef9 [Kousuke Saruta] Merge branch 'master' of git://git.apache.org/spark into SPARK-5559
8212e42 [Kousuke Saruta] Modified default port used in FlumeStreamSuite from 23456 to random value