Published Kafka-assembly JAR was empty in 1.3.0-RC1
This is because the maven build generated two Jars-
1. an empty JAR file (since kafka-assembly has no code of its own)
2. a assembly JAR file containing everything in a different location as 1
The maven publishing plugin uploaded 1 and not 2.
Instead if 2 is not configure to generate in a different location, there is only 1 jar containing everything, which gets published.

Author: Tathagata Das <tathagata.das1565@gmail.com>

Closes #4753 from tdas/SPARK-5993 and squashes the following commits:

c390db8 [Tathagata Das] Fix assembly jar location of kafka-assembly