Config variable AWS_REGION now seems required

[jucor]

Expected behavior:
Running with the default example.env (copied to .env) should allow to run polis.

Actual behavior:
When copying example.env to .env and running with make as per README.md, got an error complaining of an empty region, and the polis server not starting.

Setting:

AWS_REGION=us-east-1 
AWS_ACCESS_KEY_ID=dummy-key
AWS_SECRET_ACCESS_KEY=dummy-secret 

in .env then allowed the server to actually start.

Additional context:
I suspect this might be due to the new code that uses AWS DynamoDB to store the topics, and might not have a graceful fallback if no region is specified.

Since we still want polis to be runnable locally without AWS setup, I suggest one of two mitigations:

• find where the graceful fallback is needed and add it, to support empty AWS_REGION :)
• or at least modify example.env with a region, so polis still runs out of the box.

Further question:
This make me wonder: now that we seem to have a dependency on DynamoDB for the topics, do we have any other places that will break if someone has not set up their AWS in some specific way, or do we have a graceful fallback?
If not, should we document in the README what AWS setup is needed?