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