Use the correct docker server ip address for ssh

Honor DOCKER_HOST envvar, used with boot2docker
Use the IpAddress field from docker info otherwise, `[Ports][22/tcp][0][HostPort]` is the
bind address and can be 0.0.0.0

The current implementation is trying to ssh connect to 0.0.0.0 for me (using boot2docker on OS X)

My NetworkSettings from docker info

```
  "NetworkSettings": {
    "IPAddress": "172.17.0.47",
    "IPPrefixLen": 16,
    "Gateway": "172.17.42.1",
    "Bridge": "docker0",
    "PortMapping": null,
    "Ports": {
      "22/tcp": [
        {
          "HostIp": "0.0.0.0",
          "HostPort": "49155"
        }
      ]
    }
  },
```

I have tested it with https://github.com/maestrodev/puppet-maven

    BEAKER_set=centos-65-x64-docker rspec spec/acceptance

HostIp is the address Docker binds the port to. In my case it returned 0.0.0.0 (bound to all interfaces)

Seems the correct way to get the ip is using IpAddress
https://github.com/dotcloud/docker/issues/2509#issuecomment-27577364
http://stackoverflow.com/questions/17157721/getting-a-docker-containers-ip-address-from-the-host