Correct comment to not be driver-specific
Correct comment to not be driver-specific

The comment mentioned Fernet tokens specifically, but the situation
is going to apply any time the driver doesn't persist the token.
The manager is common code and shouldn't depend on the driver.

Change-Id: I21c104b6256d9718c233d0f44d4c06c5a1ace6c9

mahak/keystone,cernops/keystone,openstack/keystone,dims/keystone,mahak/keystone,maestro-hybrid-cloud/keystone,ajayaa/keystone,ilay09/keystone,dims/keystone,rajalokan/keystone,ilay09/keystone,ajayaa/keystone,klmitch/keystone,klmitch/keystone,himanshu-setia/keystone,mahak/keystone,himanshu-setia/keystone,cernops/keystone,ilay09/keystone,openstack/keystone,maestro-hybrid-cloud/keystone,openstack/keystone,rajalokan/keystone,rajalokan/keystone