Update botocore initializer to allow region autodiscovery

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Referencing this: https://github.com/DavidMuller/aws-requests-auth/blob/bd638ea84903e4c2f482dbade610eb978c0a240e/aws_requests_auth/boto_utils.py#L34

This call requires host, region, and service to be provided. Since it uses botocore functionality, it should be able to capture region from the environment if not explicitly supplied. It does have to be override-able, but shouldn't be required by default.

Making region optional is a potentially breaking change, since the order would likely also change.