Adds Hortonworks Data Platform element to savanna-image-elements

This patch adds a new element, "hortonworks-hdp" to the
  savanna-image-elements project.  This element can be used to create
  Virtual Machine images that have the Hortonworks Data Platform (HDP)
  installed by default.
The element includes support for installing version 1.3 and 2.0 of the
  platform, and the README gives more details on how to configure this.
The patch also includes a bash script that creates several CentOS
  images with HDP pre-installed.  This script can either be used to
  generate the images as they are currently configured, or can be used
  as an example of how to use diskimage-builder to create qcow2 images
  with HDP.
This patch only supports generating CentOS images, and we could
  potentially add more supported platforms as time permits.

Implements: blueprint add-hdp-images-to-savanna-image-elements

Change-Id: I7108851f760147d379e82078cf390d7ff13d8898