/usr/share/tripleo-heat-templates/environments/updates/update-from-overcloud-compute-hostnames.yaml is in python-tripleo-heat-templates 5.2.0-1.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 | parameter_defaults:
ComputeHostnameFormat: '%stackname%-compute-%index%'
|