Supported AWS regions in #supported_regions helper

The supported_regions helper is a wrapper around an instance variable
so that in future, fog users can self-modify the list as data
to support newly available services in regions, or newly available
regions; without waiting for fog version releases.
Hmm. It seems appealing, but historically I think the timing of new region support has varied/lagged for different services. That said, maybe we could still have a core version that just gets extra stuff (if any) merged on top in this context. I think that would provide a clearer provision for the known mixed use case, what do you think?