SPARK-5728 [STREAMING] MQTTStreamSuite leaves behind ActiveMQ database files

Use temp dir for ActiveMQ database

Author: Sean Owen <sowen@cloudera.com>

Closes #4517 from srowen/SPARK-5728 and squashes the following commits:

1d3aeb8 [Sean Owen] Use temp dir for ActiveMQ database

external/mqtt/src/test/scala/org/apache/spark/streaming/mqtt/MQTTStreamSuite.scala

@@ -93,6 +93,7 @@ class MQTTStreamSuite extends FunSuite with Eventually with BeforeAndAfter {
 
   private def setupMQTT() {
     broker = new BrokerService()
+    broker.setDataDirectoryFile(Utils.createTempDir())
     connector = new TransportConnector()
     connector.setName("mqtt")
     connector.setUri(new URI("mqtt:" + brokerUri))