support multiple image formats and enable docker driver

this patch allows you to set the image format in config
file and then use either docker or qcow.

There are a few oustanding patches for novadocker driver
but I have it working from my fork which can be used from
https://github.com/rackerlabs/vagrant-solum-dev/tree/docker_driver

Change-Id: Ia8ad6519efa0e0c3a32a787a7ef78895110871f9
Merge "support multiple image formats and enable docker driver"