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.
According to http://docs.aws.amazon.com/general/latest/gr/rande.html#elasticache_region ap-southeast-2 should be in this list. That means all of the lists in this PR are the same. Should we move them up a level to a common constant in `lib/fog/aws.rb`?