Comment 4 for bug 1907519

Resource class is already supported:
https://docs.openstack.org/project-deploy-guide/tripleo-docs/latest/provisioning/baremetal_provision.html#instance-and-defaults-properties

For example:
 - name: Controller
   count: 3
   defaults:
     resource_class: Controller

You can also use capabilities profile.

For example:
 - name: Controller
   count: 3
   defaults:
     profile: Controller