Add support for docker proxy

Configure docker to use proxies if it is installed.

Docker daemon needs http_proxy environment when behind proxies in a docker config file as follows:

- [RHEL](http://docs.docker.io/installation/rhel/)
  - `/etc/sysconfig/docker` 
- [Ubuntu](http://docs.docker.io/installation/ubuntulinux/)
  - `/etc/default/docker` ( < Ubuntu 14.04)
  - `/etc/default/docker.io` ( >= Ubuntu 14.04 )
- [boot2docker](https://github.com/boot2docker/boot2docker/blob/master/rootfs/rootfs/usr/local/etc/init.d/docker)
  - [x] `/var/lib/boot2docker/profile`

I could not find any official documents docker for proxy.
See following references.
[docker issues 1852 ](https://github.com/dotcloud/docker/issues/1852)
[Stack Overflow: Cannot download Docker images behind a proxy](http://stackoverflow.com/questions/23111631/cannot-download-docker-images-behind-a-proxy)