Document reasoning about aws region

[dkropachev]

There is a problem that we provide bogus AWS region name to the aws client. If client have tracing/debuging or anything else that looks at aws region it can create confusion or missbehavior.

We need to document reasoning behind bogus AWS region and a way to set it properly.

Example case: https://github.com/scylladb/alternator-load-balancing/issues/30