Report the iso path used by the vm to help debug possible support issues

Follow-on for #287 

While testing I was tripped up by the fact that one of my boxes has a `~/.boot2docker/profile` I'd forgotten - and without a b2d way to find out what the iso file path really is, we'd have to resort to asking users to tell us (rather than c&p), which is fraught with extra round trips.

at the same time, I've formatted the json so its readable.

@tianon @gmlewis 
`json.MarshalIndent` is what you're thinking of, I believe: http://golang.org/pkg/encoding/json/#MarshalIndent

(and I agree)