host only networking as per boot2docker PR 198

It works, creates a new network when needed, or reuses one if the cfg is the same as the one specified.

We need _something_ to get the IP address out of the VM.

- using a scripted ssh isn't very portable - expect on windows?
- perhaps just add a fingerd to boot2docker that just spits out some status?
- or ... is this information available via the docker API?