configurable keystone-token-flush cronjob schedules

[Alvaro Uria]

By default, keystone-token-flush cronjob runs once every hour (0 * * * *).

In the past, we have had trouble with long transactions locking the keystone.token table, degrading the service due to token timeouts and restoring normal operation by killing such transaction in mysql.

I would like to suggest a new config parameter to be able to run the cronjob every N minutes (ie. 10 minutes). Although token-expiration would be configured to 3600 or more, there would be tokens to delete within the hour, reducing the workload on larger clouds.

Sample:
juju config keystone token-cron-schedule='*/10 * * * *'

  token-cron-schedule:
    default: true
    description: |
      Cron schedule used to run keystone-manage token_flush.
      If unset, it will run once an hour
    type: string
    value: '0 * * * *'