[SPARK-7050] [BUILD] Fix Python Kafka test assembly jar not found issue under Maven build

 To fix Spark Streaming unit test with maven build. Previously the name and path of maven generated jar is different from sbt, which will lead to following exception. This fix keep the same behavior with both Maven and sbt build.

```
Failed to find Spark Streaming Kafka assembly jar in /home/xyz/spark/external/kafka-assembly
You need to build Spark with  'build/sbt assembly/assembly streaming-kafka-assembly/assembly' or 'build/mvn package' before running this program
```

Author: jerryshao <saisai.shao@intel.com>

Closes #5632 from jerryshao/SPARK-7050 and squashes the following commits:

74b068d [jerryshao] Fix mvn build issue

Author: jerryshao

external/kafka-assembly/pom.xml

@@ -58,6 +58,7 @@
       <artifactId>maven-shade-plugin</artifactId>
       <configuration>
         <shadedArtifactAttached>false</shadedArtifactAttached>
+        <outputFile>${project.build.directory}/scala-${scala.binary.version}/spark-streaming-kafka-assembly-${project.version}.jar</outputFile>
         <artifactSet>
           <includes>
             <include>*:*</include>