Periodic image.exists event generation

Adds a script which can be run periodically to generate an image.exists
    for each image which was active at any point during the past audit
    period. Two type of images are considered as having existed during
    the given audit period. First are active images that were created
    before or during the audit period. Second are images that were
    active but were deleted during the audit period. To determine if
    and image was or is active, we select based on location having been
    set and whether created_at/deleted_at are in range.

Partially implements: bp:glance-notifications-deux

Change-Id: Ic53ccb4c1c806db6eb22143bc717f01fc3e08763