Techdebt: Remove dependency on get_available_regions

[bblommers]

Moto uses, for the vast majority of services, the botocore.get_available_regions-method to determine in what regions a particular service is available.

This method was always a bit flaky - it would take along time for new services to be available, and every now and then a new release would break the list for an existing service.

AWS' recommendation is to use the SSM parameter store instead, to determine per-region availability - see aws/aws-sdk#206 (comment)

Moto should investigate whether we can also switch to using this method. The data is available in Moto, and it is updated regularly, so I assume it will be possible - just a potential performance issue to have to read the JSON file first thing.