Remove call to EC2:DescribeRegions API

[zmoog]

What does this PR do?

Remove the call to the EC2:DescribeRegions API.

Why is it important?

We aim to increase performance and avoid throttling.

As of today, the Lambda runtime can only trigger the function from a (CloudWatch) log group in the same region as the function.

Since the log group and function region are the same, we can use the lambda function region to look up ¹ the input in the configuration and avoid matching with all the existing regions.

Checklist

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have made corresponding change to the default configuration files
• I have added tests that prove my fix is effective or that my feature works
• I have added an entry in CHANGELOG.md

Footnotes

1. We need a region to lookup the input in the config file because the log event from cloudwatch does not contain the region. ↩