Fixed #675: Added an ENV/Option to Configure USE_SIGV4

Currently the new AWS SIGNATURE V4 wasn't supported by the docker_registry.
I created a ENV Variable called AWS_USE_SIGV4 which can either be true or false.
This could also be set using the Configuration S3_USE_SIGV4 inside a config file
This would probably be more appropriate to default to None `s3_use_sigv4: _env:AWS_USE_SIGV4` since when it's not True, nothing is set instead of setting it to False.