Allow docker publish to use an installed docker cli

Setting the docker_version to "local" or leaving it out in the docker publish plugin will skip installing the docker client and attempt to use one that is already installed.

Would it make sense to ship the base build images with docker installed, similar to git etc?
instead of using an extra flag, don't you prefer to install docker binary only if docker isn't available in the system?

see:
```bash
$ type -p docker ; echo $?
/sbin/docker
0
$ type -p nonexistent ; echo $?
1
```

so the line that download and install docker can be changed to:

```
f.WriteCmd("type -p docker || wget -qO- https://get.docker.io/builds/Linux/x86_64/docker-" d.DockerVersion + ".tgz |sudo tar zxf - -C /")