Enable the scaling up of nodes in a cluster

Enabled the ability to add nodes to a cluster.  The ability to delete
nodes will be added in a future commit once Ambari supports the
deletion of hosts from a cluster.

Fixes: bug #1216088

Conflicts:
	savanna/plugins/hdp/ambariplugin.py
	savanna/plugins/hdp/hadoopserver.py

Change-Id: I5bd5bf1b55412fbebbc5d9eb84cccc57e4b1824c
(cherry picked from commit f2ef149aac5568c2b41f8206cfd467984ba32198)