This adds AggregateCacheNotification, related payload, and code in
conductor to emit this per-compute with progress information. This
also adds a "progress" phase to NotificationPhase, which allows for
start..progress..progress..end information for a single operation
(cache_images in this case).
Related to blueprint image-precache-support
Change-Id: I69ae26d4caf4b56ab2c4864455bfe9b5b736dbf3
Dan Smith