[spark] Migrate Livy to external respository

1. Remove Hue's Livy repo and point to https://github.com/cloudera/livy
2. Check that https://github.com/cloudera/livy/blob/master/README.rst works
3. Change README to http://gethue.com/how-to-use-the-livy-spark-rest-job-server-for-interactive-spark-2-2/
4. Update blog pots

And investigate follow-up of HUE-3661 and how to select an existing session, or create a custom session from the beginning.