Add support for building QEMU images from pre-existing image files

If the `disk_image` parameter is set to true treat the ISO location as the source of the disk image, copy it to the destination, resize and boot it.

I've included an example configuration that builds from the latest Ubuntu 14.04 Cloud image [here](https://gist.github.com/insom/2974a359121478766e84) - it requires an index.html file be put in the `httpdir/latest/meta-data` directory because of how CloudInit validates that it's talking to a good source of configuration data.