Adds SSH support to HDP plain VM image

This patch adds the "ssh" element to the list of elements
  used for the generation of the HDP "plain" image.  This image
  does not contain any HDP installed packages, but is configured
  for deploying HDP Hadoop clusters.

This patch also adds the disable-firewall element to the list of
  elements for the generation of the HDP "plain" image.  Recent testing
  showed that the plain image was not supporting cluster startups
  since Apache Ambari requires that the iptables service be turned
  off for nodes in an HDP cluster.

Closes-Bug: #1306726

Change-Id: I3924232495b1f11c61b613d0fa7c6ac669d1b8b8
Merge "Adds SSH support to HDP plain VM image"