Be able to set a global default service region

DEFAULT_SERVICE_REGIONS lets you set a default service region per endpoint. This is rather roundabout and awkward when you want to point all new logins to a specific region.

Adding a new DEFAULT_SERVICE_REGION value would look too similar to DEFAULT_SERVICE_REGIONS so we should instead add a global key option to DEFAULT_SERVICE_REGIONS that lets it be used as a global fallback when no other key matches.

This key would be '*' and its addition would be entirely backwards compatible.