Bug 1644801

Description of problem:
In order to make the repository update easier using the redhat-subscription role when updating from osp12 to osp13 it would be needed to have available the package in osp12 repositories.
Currently, if we would like to use the Rhsm service to upgrade from osp12 to osp13 an error would be printed out as the redhat-subscription role is not available in the overcloud nodes (the role has been added in osp13).
Some effort have been done to make use of the old rhel-registration script inside the Rhsm service upgrade_taks, mapping the new variables to the old:
https://review.openstack.org/#/c/611879/ 

However, the effort to make it work is pretty big compared to having the option of using the role which already handles the repository switching. Also, as seen in the review this approach would force us to include a task in host_prep_tasks to verify if the role is available or not (as when running "overcloud upgrade prepare" the host_prep_tasks are executed and the role is not available, the same case stated above).

If the role would be available already in osp12, we could suggest in the documentation to start making use of it when upgrading (linking to the migration to Rhsm documentation https://access.redhat.com/documentation/en-us/red_hat_openstack_platform/13/html/advanced_overcloud_customization/ansible-based-registration#switching-to-the-rhsm-composable-service ) appending the rhsm.yml template in the resource_registry. 


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