Add DEA::ExitMessage (for droplet.exited)

Change-Id: I9fa8929174109d1d6553b02d3e2fe6f720d3f271
Add DropletUpdatedMessage

Change-Id: Ifcabf7eea6e5c6d914694fa002072aef3ea40477
Add HealthManager::StatusRequest

Change-Id: I9dc866845ae565aab50636164258f748f1077eb3