This adds a simple purge command to nova-manage. It either deletes all
shadow archived data, or data older than a date if provided.
This also adds a post-test hook to run purge after archive to validate
that it at least works on data generated by a gate run.
Related to blueprint purge-db
Change-Id: I6f87cf03d49be6bfad2c5e6f0c8accf0fab4e6ee
Dan Smith
ae241cc68f