The ruby fog library supports a lot of cloud providers. It started as simple gem that got very complex due to to the increasing support. OpenNebula was also supported but, the plugin got a bit stale and its continuity got jeopardized, but now its updated and merged into fog upstream.
You can check the fog website with the new official support for OpenNebula. The repository is at https://github.com/fog/fog-opennebula/blob/master/README.md. Contributions are welcome !!