Allow for option for unsigned request

[treble-maker123]

When developing locally, I have a local instance of DynamoDB running that does not looking at the signature of the request. However, we still need to have valid AWS credentials on our local computer in order for PynamoDB to work, otherwise it would throw,

botocore.exceptions.ClientError: An error occurred (InvalidClientTokenId) when calling the AssumeRole operation: The security token included in the request is invalid.

The configuration should have the option to allow for unsigned request (e.g. boto/boto3#1200) so we don't need to maintain valid credentials on our computers to test something that does not require any credentials.

Happy to raise a PR if you guys think this is helpful. I think it's just a matter of being able to pipe that option to https://github.com/pynamodb/PynamoDB/blob/master/pynamodb/connection/base.py#L576-L581