Allow restricting Sydent to use by a specific list of Homeservers

[benbz]

As discussed with @erikjohnston and others, we need to be able to restrict the homeservers that a given Sydent deploy works with. The suggestion is that there is internal code for this and a config option needs exposing and testing to occur.

Ideally this would be a list of homeservers, but for now a single homeserver is acceptable