Set keystone token backend to sql.

Because the table could grow forever, also add a cron job to flush tokens
every two hours.

Due to bug #1188378 we need to use percona-toolkit's pt-archiver to
clean out the table on mysql.

Fixes bug #1188301

Change-Id: I90aac347993ee741231fbfb99a32dc6e3f3a75ec
Merge "Set keystone token backend to sql."