Adds DataNode decommissioning support to HDP Plugin

This patch adds support for decommissioning HDFS datanodes
  during a de-scaling operation in Sahara.  The HDP
  plugin will now make requests to the Ambari server
  in the cluster to decommision any DataNodes deployed
  on the instances that are being shutdown.  This
  change is required to allow HDFS to gracefully
  handle the case where a set of DataNodes are
  being shutdown due to a scaling change.
This patch adds this support for HDP 2.0.6 only.
A new method has been added to the AmbariClient class
  for HDP 2.0.6 to implement the set of REST API
  calls to Ambari Server to start and monitor the
  process of decommissioning HDFS DataNodes on
  the machines that will be de-scaled.
The AmbariPlugin's implementation of decommission_nodes()
  has been modified to use the HDP 2.0.6 AmbariClient to
  handle the decommissioning (in the case of 2.0.6).
  The plugin will throw an exception if this call is
  attempted for an HDP 1.3.2 cluster.

Closes-Bug: #1335923

Change-Id: I49c8d1fef8a0916563c047b55d01054db2849a18
Merge "Adds DataNode decommissioning support to HDP Plugin"