identity: flush tokens every days at midnight

Install a crontab to flush expired keystone tokens every days at
midnight.
The reason is that we are using MySQL backend for token storage and when
having a big deployment, Database could grow faster than we expect.

While we find another backend for Keystone which scale, let's create a
crontab which will avoid a lot of issues in production :-)
Merge pull request #474 from enovance/flush-tokens

identity: flush tokens every days at midnight