Store keystone tokens in mysql.

Keystone would commonly become unresponsive when using
the memcached token backend.  This is suspected to be due
to known issues with how keystone handles token expiration
from memcached, which causes the keystone process to peg
cpu usage iterating through memcached for every request.

This change

  - switches keystone to use the mysql token backend

  - adds an hourly cron job to remove stale tokens from the
    keystone.token table, so that the table does not grow without
    bound.