Update KafkaEmbedded so that Kafka itself can grab a random port

[philwebb]

Currently KafkaEmbedded uses this logic to grab a random port. Unfortunately this isn't always reliable because the OS can reassign the port before it's used. This causes exceptions like this:

kafka.common.KafkaException: Socket server failed to bind to localhost:49270: Address already in use.
	at kafka.network.Acceptor.openServerSocket(SocketServer.scala:331)
	at kafka.network.Acceptor.<init>(SocketServer.scala:256)
	at kafka.network.SocketServer$$anonfun$startup$1.apply(SocketServer.scala:97)
	at kafka.network.SocketServer$$anonfun$startup$1.apply(SocketServer.scala:89)
	at scala.collection.mutable.ResizableArray$class.foreach(ResizableArray.scala:59)
	at scala.collection.mutable.ArrayBuffer.foreach(ArrayBuffer.scala:48)
	at kafka.network.SocketServer.startup(SocketServer.scala:89)
	at kafka.server.KafkaServer.startup(KafkaServer.scala:229)
	at kafka.utils.TestUtils$.createServer(TestUtils.scala:133)
	at kafka.utils.TestUtils.createServer(TestUtils.scala)
	at org.springframework.kafka.test.rule.KafkaEmbedded.before(KafkaEmbedded.java:200)
	at org.springframework.boot.actuate.kafka.KafkaHealthIndicatorTests.startKafka(KafkaHealthIndicatorTests.java:96)
	at org.springframework.boot.actuate.kafka.KafkaHealthIndicatorTests.kafkaIsUp(KafkaHealthIndicatorTests.java:55)
	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
	at java.lang.reflect.Method.invoke(Method.java:498)
	at org.junit.runners.model.FrameworkMethod$1.runReflectiveCall(FrameworkMethod.java:50)
	at org.junit.internal.runners.model.ReflectiveCallable.run(ReflectiveCallable.java:12)
	at org.junit.runners.model.FrameworkMethod.invokeExplosively(FrameworkMethod.java:47)
	at org.junit.internal.runners.statements.InvokeMethod.evaluate(InvokeMethod.java:17)
	at org.junit.internal.runners.statements.RunAfters.evaluate(RunAfters.java:27)
	at org.junit.runners.ParentRunner.runLeaf(ParentRunner.java:325)
	at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:78)
	at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:57)
	at org.junit.runners.ParentRunner$3.run(ParentRunner.java:290)
	at org.junit.runners.ParentRunner$1.schedule(ParentRunner.java:71)
	at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:288)
	at org.junit.runners.ParentRunner.access$000(ParentRunner.java:58)
	at org.junit.runners.ParentRunner$2.evaluate(ParentRunner.java:268)
	at org.junit.runners.ParentRunner.run(ParentRunner.java:363)
	at org.apache.maven.surefire.junit4.JUnit4Provider.execute(JUnit4Provider.java:369)
	at org.apache.maven.surefire.junit4.JUnit4Provider.executeWithRerun(JUnit4Provider.java:275)
	at org.apache.maven.surefire.junit4.JUnit4Provider.executeTestSet(JUnit4Provider.java:239)
	at org.apache.maven.surefire.junit4.JUnit4Provider.invoke(JUnit4Provider.java:160)
	at org.apache.maven.surefire.booter.ForkedBooter.invokeProviderInSameClassLoader(ForkedBooter.java:373)
	at org.apache.maven.surefire.booter.ForkedBooter.runSuitesInProcess(ForkedBooter.java:334)
	at org.apache.maven.surefire.booter.ForkedBooter.execute(ForkedBooter.java:119)
	at org.apache.maven.surefire.booter.ForkedBooter.main(ForkedBooter.java:407)
Caused by: java.net.BindException: Address already in use
	at sun.nio.ch.Net.bind0(Native Method)
	at sun.nio.ch.Net.bind(Net.java:433)
	at sun.nio.ch.Net.bind(Net.java:425)
	at sun.nio.ch.ServerSocketChannelImpl.bind(ServerSocketChannelImpl.java:223)
	at sun.nio.ch.ServerSocketAdaptor.bind(ServerSocketAdaptor.java:74)
	at sun.nio.ch.ServerSocketAdaptor.bind(ServerSocketAdaptor.java:67)
	at kafka.network.Acceptor.openServerSocket(SocketServer.scala:327)
	... 38 more

I think it's possible to set the Kafka config with a port of 0 to have the server itself start on a random port.