registry: Introduce DOCKER_REGISTRY_SSL_NO_VERIFY environment variable

Currently docker registry http request may block by some gateway
(for example, company web proxy), because of the certificate issue.

[root@localhost docker]# docker pull ubuntu
Pulling repository ubuntu
2014/05/15 15:51:24 Get https://index.docker.io/v1/repositories/ubuntu/images: x509: certificate signed by unknown authority

This patch introduce a new environment variable DOCKER_REGISTRY_SSL_NO_VERIFY
to control secure certificate verify. The default value is false.

The environment variable name comes from git: GIT_SSL_NO_VERIFY
(Similarly, wget and curl both have env variable or option to control secure
verify)

Docker-DCO-1.1-Signed-off-by: Jovi Zhangwei <jovi.zhangwei@gmail.com> (github: ktap)