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
I think you should warn when the host isn't set, but still set use-sigv4.  Just remove the else and indent out the lines below.
if ..host is None:
  logger.warn(..)
boto.config.add_section..
boto.config.set('s3', 'use-sigv4', 'True')

I think its better to have the config file do as told, since we may not know of a valid reason to set this while the config isn't specified.