Add ability to port-forward TCP docker service to host machine

Based on information from here:

http://coreos.com/docs/launching-containers/building/customizing-docker/#enable-the-remote-api-on-a-new-socket

I use boot2docker right now, but would like to use CoreOS as my target  platform for building docker containers.  This change mimics what boot2docker does, forwards docker's service port to the host machine so that you can use the docker tools if you're not using Linux as a host workstation.

e.g.:

```
~/src/coreos-vagrant master ✔
🍺   uname -s
Darwin

~/src/coreos-vagrant master ✔
🍺   docker info
Containers: 0
Images: 0
Storage Driver: btrfs
Execution Driver: native-0.1
Kernel Version: 3.14.1+
```