supporting for CloudFront storage driver

[stagegrowth]

Registry supports cloudfront too.
Could you add middleware field at config.yml.ctmpl and wrapper envs to _env

This is a part of yaml in registry server using cloudfront

middleware: storage: - name: cloudfront options: baseurl: http://${url_name}.cloudfront.net keypairid: ${key_pair} privatekey: ${cloudfront_key} duration: ${duration_time}