Add docker RHEL7 support

<del> This is a work in progress. </del>
I have found that vagrant-proxyconf docker support does not work on RHEL 7.

- On RHEL 6:
  -  docker proxy config is in `/etc/sysconfig/docker` and it is 'called' from [init](https://github.com/dotcloud/docker/blob/b9f66049fa4d0dc2cdc0a430fe14029214456611/contrib/init/sysvinit-redhat/docker#L31).
  - it should be `export http_proxy=http://myproxy:3128/`
- On RHEL 7:
  - docker proxy config is in `/etc/sysconfig/docker` but it is not 'called' but just 'evaluated' by systemctl.
  - it should be `http_proxy=http://myproxy:3128/`

I need to do:
- [x] implementation
- [x] tests
  - [x] CentOS 7.0
  - [x] CentOS 6.5
  - [x] Ubuntu 14.04
  - [x] boot2docker

------
On my Mac I tested with this [Vagrantfile](https://gist.github.com/otahi/25021510386a12e765bd).
(I had already installed some boxes)

Then I have got the result, that means dockers have correct `http_proxy` environment value.

```
$ for vm in centos7 centos6 ubuntu14.04 boot2docker ; do echo -n $vm, ; vagrant ssh $vm -c 'sudo cat /proc/`pgrep docker|head -n1`/environ | grep -c http_proxy' 2&> /dev/null ; done
centos7,1
centos6,1
ubuntu14.04,1
boot2docker,1
$
```