First cut at Nova plugin

A plugin that uses Nova Image Builder to create a base image using Nova on a remote Openstack infra instead of using Oz and libguestfs to do it locally.

This is the first cut and there are things that could be improved to make this less weird. Things like:

- pointing to the glance image in the base image body file
- somehow mapping all of the IDs that get created during build
Merge pull request #332 from redhat-imaging/imagefactory-324

First cut at Nova plugin