Drop need for credentials if endpoint is localhost

[colinlouie]

Interestingly enough I found that AbstractInMemoryTest.scala creates an in-memory Dynamo instance, setting the system property aws.dynamodb.endpoint to http://localhost:8000.

This was tested on macOS Catalina 10.15.7, running DynamoDB with the AWS Docker image.

shell$ sbt test
  :
[info] Run completed in 15 seconds, 577 milliseconds.
[info] Total number of tests run: 21
[info] Suites: completed 8, aborted 0
[info] Tests: succeeded 21, failed 0, canceled 0, ignored 0, pending 0
[info] All tests passed.
[success] Total time: 21 s, completed Feb 27, 2021 12:03:36 PM