enabling tokens flushing using cron

This patch is for configuring a flush token cron when using database as
backend for tokens storage.
While potentially useful for auditing in production environments,
the accumulation of expired tokens will considerably
increase database size and may decrease service performance,
particularly in test environments with limited resources.
It's recommanded to create a crontab to flush tokens every hour.

Change-Id: Icd819cc95b3440f43c5b588f9f2dd6ca47b25a71
Signed-off-by: Emilien Macchi <emilien.macchi@enovance.com>
(cherry picked from commit 6aa7a17941056a7de2a7bdeeb99024b5c714f882)