RHSM isn't managed after an initial deployment

[Emilien Macchi]

RHSM script works fine for initial deployments, during a stack CREATE:
https://github.com/openstack/tripleo-heat-templates/blob/master/extraconfig/pre_deploy/rhel-registration/rhel-registration.yaml#L122

Which means that during a stack UPDATE, the script isn't executed.
There are cases where running the script would be useful:
- scale-out compute nodes (new nodes need registration)
- change in the registration parameters (new pools, or new account)

We'll need to make the registration script idempotent, so we can run it at a stack UPDATE.