If your using config drive it is by default stored in /var/lib/nova/instances/VMUUID
When you move to liberty everything is stored in the ceph vms pool along to maintain high availability access.
If you have machines that do not use ceph you will need to import their config drives.
rbd import -p vms --id cinder /var/lib/nova/instances/VMUUID/disk.config VMUUID_disk.config