Add Spark plugin to Sahara

Integrate the Spark plugin in Sahara. It adds the capability of
provisioning Spark clusters with (Cloudera) HDFS. The plugin
assumes the use of VM images generated with the Savanna
diskimage-builder and the '-p spark' option.

Implements: blueprint spark-plugin

Wiki: https://wiki.openstack.org/wiki/Sahara/SparkPlugin

This code is running on our Bigfoot research cluster.

Change-Id: Ic105f7de64248bdfb05879ededf35503bc04925b
Merge "Add Spark plugin to Sahara"