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
usg_sigv4 typo at the end.

I also think the last part could just be "boto_host needs to be set or use_sigv4 will be ignored by boto."  No need for the Warning or ().