Startup handler for JClouds.
The sole purpose of this handler is to delete/suspend stale supplemental
instances which might have been left running when jenkins was forcibly restarted or
shutdown while running a job that uses JCloudsBuildWrapper.
In order to do so,
TerminateNodes
persists a list of nodes to shutdown before it is attempting the actual delete/suspend.
After successfully handling all nodes, the persisting xml file is removed. During a
hard shutdown/restart, this process is usually aborted prematurely and the xml file
remains. This handler then picks those files up at the next jenkins startup and
completes the operation.