Create `docker hosts inspect` command

Add a `docker hosts inspect foo` command.

Very useful for debugging / the times I just want to see some property of the instance such as IP Address.

e.g.

```sh
{
    "DriverName": "ec2",
    "Driver": {
        "Auth": {
            "AccessKey": "ANACCESSKEY",
            "SecretKey": "HAYOUDIDNTREALLYTHINKIDPOSTTHISDIDYOU"
        },
        "Endpoint": "https://ec2.us-west-1.amazonaws.com",
        "ImageId": "ami-27939962",
        "InstanceId": "i-4c30a686",
        "InstanceName": "docker-host-aa451ae9208d5a6df9bda0f8e428bb1c791ca6567346a576ddd82ce3860388a5",
        "InstanceType": "t1.micro",
        "KeyName": "docker-host-aa451ae9208d5a6df9bda0f8e428bb1c791ca6567346a576ddd82ce3860388a5-key",
        "PublicDnsName": "ec2-54-183-203-60.us-west-1.compute.amazonaws.com",
        "IPAddress": "54.183.203.60",
        "SecurityGroup": "docker-hosts",
        "Region": "us-west-1",
        "Username": "ubuntu"
    }
}
```

Things to think about: in the future we might want to have a ``no_display`` tag on the driver struct so that secrets etc. (even though they're kept on disk) aren't displayed in `inspect` when, say, you're showing your buddy something as he looks over your shoulder- or presenting to an audience of 200 people and being recorded.  I think that would entail walking the struct and wiping those fields before Unmarshaling/printing them though.

Docker-DCO-1.1-Signed-off-by: Nathan LeClaire <nathan.leclaire@gmail.com> (github: )