get_hosting_services was a generator returning tuples of (id, cls) for

each HostingService. This was to work around not having a way to access

the ID from a HostingService class.
This change fixes this by assigning the ID at registration time to the

class, allowing any consumer of the class to get the ID. This simplifies

get_hosting_services(), and is also needed for the new HostingService

API resource.

Unit tests pass.
Used the ID in another change successfully.