参考：
https://docs.openstack.org/keystone/queens/admin/identity-performance.html
清楚过期的Token.
Pruning expired tokens from backend storage¶
Using a persistent token format will result in an ever-growing backend store. Keystone will not remove, or prune, tokens from the backend even after they are expired. This can be managed manually using keystone-manage token_flush, which will purge expired tokens from the data store in batches. Diligently pruning expired tokens will prevent token bloat.
This optimization is not necessary for deployments leveraging Fernet tokens, which are non-persistent in nature.
[root@controller ~]# keystone-manage token_flush
[root@controller ~]#
Queens默认采用的是Fernet,所以不需要运行这个命令。