Automatically fetch AWS regions

[patrislav]

Instead of hardcoding a map of AWS region codes and their names, we should consider to instead fetch them directly from AWS. There are at least two different options of dealing with this:

1. Having a go generate target that fetches the regions and generates a file based on them,
2. Whenever starting the ingester, fetching the regions live from AWS

Considering how infrequently the list changes, option 1 might be less obtrusive.

See: https://github.com/cycloidio/poc/pull/11#discussion_r498819645