Keystone: Move to SQL backend for Token storage

Using Memcache as storage backend for Tokens in Keystone, we hit this
bug: https://bugs.launchpad.net/keystone/+bug/1242620

It generates some bugs when running Tempest framework and also failures
in our QA process.

After some investigation, it seems the bug will not be fixed in Havana
and we should switch to SQL backend. Another work will be done
also on the MySQL keystone tables to avoid useless replication for
large scale deployments.