registry: Introduce --insecure daemon flag to control secure certificate verify with registry

Currently many users cannot pull registry image behind corporate firewall,
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 daemon flag "--insecure" to control secure
certificate verify. The default value is false.

Note that this global daemon flag will affects *all* hosts, it's also possible
make sense allow per-host configuration in future.

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