Steps to reproduce:

* Deploy a cloud
* Add another controller to the inventory
* Deploy to the new controller using --limit:

kolla-ansible deploy --limit new-controller

Expected results:

The new controller uses the cluster's existing fernet keys.

Actual results:

New fernet keys are generated on the new controller, and pushed out to
the existing controllers. This invalidates tokens created from those
keys.

This change prevents the above scenario from happening, by failing the
deployment if there are no hosts with existing Ferney keys to
distribute, and not all Keystone hosts are in the target host list.

Closes-Bug: #1891364

Change-Id: If0c0e038b77fc010a3a017f9841a674d53b16457