Always use the SQL backend for tokens.

The KVS has a number of drawbacks:

* Requires load balancer to persist connections to a single keystone
 server by token
* Memory will grow out of control until keystone server is restarted
*  At some point kvs lookups get very slow because there are millions of
*  keys in the dict.
* Process restart invalidates all tokens.

see e.g. https://bugs.launchpad.net/tripleo/+bug/1188301