Let's discuss whitelisting service-types

Hello!

The user of this resource might just want to add something obscure.

[This user was trying to register](https://github.com/cloudbau/cookbook-openstack-orchestration/tree/identity-registration) the Heat service types `cloudformation` and `orchestration`, but it was not allowed.

Would you consider it better to include these into the list of allowed values instead of just letting go of restricting the allowed types?