Allow override of `aws_*` configs in global settings

[sirianni]

For testing with dynamodb-local I'm able to override the host and region using global settings, but not aws_secret_key_id and aws_secret_access_key.

As I understand it, the workarounds are either

1. Specify them on every Meta class (hardcoded in this example, would come from shared config file in a real application)

class WidgetModel(Model):
    class Meta:
        table_name = "widgets"
        aws_access_key_id = "bogus"
        aws_secret_access_key = "bogus"

2. Set them as environment variables before running the process

export AWS_SECRET_KEY_ID=bogus
export AWS_SECRET_ACCESS_KEY=bogus

(this is what pynamodb itself does for unit tests)