New OpenNebula compute resource

Hi,

we added a compute resource for OpenNebula. With this PR it is possible to create, delete, start, stop virtual machines in an OpenNebula instance.  VNC is working as well. New virtual machines are created from the available templates in the OpenNebula instance and some major values can be overridden.

Fog is used to talk to the OpenNebula XML-RPC daemon, though it is necessary to install a fog version with a OpenNebula provider (there is an existing pull request to the fog project) which is available here https://github.com/NETWAYS/fog.

Any feedback is welcome!