According to the keystone documentation [1], there is no need
to define cache directives in keystone.conf, as long as the
global caching directive is enabled (all the components are
caching by default).
[1]: https://docs.openstack.org/keystone/latest/admin/identity-caching-layer.html
Change-Id: Ibd23343691de4e15c59649f880e5e3b035a5856b