This patch introduces a new class, manila::cron::db_purge, to set up
cron job to purge soft deleted records from database.
Backport note:
This change includes the follow-up change[1] which fixed the wrong
description.
[1] ace61c29eb
Change-Id: I815a170a7fb2fc3c48bfabcfdf865d6a22f5cbe8
(cherry picked from commit 8ecb5530ba)