Periodic image.exists event generation

Implements blueprint glance-exists-notification

Adds a script which can be run periodically to generate an image.exists
notification for each image that was active at any point during the
past audit period. The following types of images are considered to have
existed during the given audit period:
-  active images that were created before or during the audit period
-  images that were active but were deleted during the audit period.
To determine if an image was or is active, we check if its location is
set and that its created_at/deleted_at values are in the audit period.

Change-Id: I69a89f110093f9bf965a289ede6f1b45ab772a0f

Ply-Patch: Periodic-image.exists-event-generation.patch