`boot2docker init` does not download the iso if it doesn't have one

[SvenDowideit]

this was lost in the multi-hypervisor refactor.

some refactoring of the cmd.go is needed as boot2docker --init up does not work - it doesn't make ssh keys, and doesn't download either - it should (optionally) call cmdInit, and B2D.GetMachine should probably have its 'init' code separated back out.

[SvenDowideit]

boot2docker init creates and invalid vm atm, so I may hack a partial fix/check.

[tianon]

Sorry, this repository is long-since deprecated in favor of Docker Toolbox (whose usage is now also discouraged in favor of Docker for Windows and Docker for Mac).