Add cron job to periodically flush expired tokens

By default, the Identity Service stores expired tokens in the database
indefinitely. 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. I added
a step to create a periodic task using 'cron' which calls
'keystone-manage token_flush' to purge expired token hourly.

Change-Id: Ie2bb4b03da25037426daa57edf084e9801a139fb
Closes-Bug: #1287962
Merge "Add cron job to periodically flush expired tokens"