cloudinit: add opennebula config format

cloudinit: add opennebula config format

This is an alternative format for cloudinit use by opennebula,
https://cloudinit.readthedocs.io/en/latest/topics/datasources/opennebula.html

but it can be also used by opennebula context scripts

https://github.com/OpenNebula/addon-context-linux
https://github.com/OpenNebula/addon-context-windows

This context scripts are simple udev trigger/bash scripts
and allow live configuration changes.

Signed-off-by: Alexandre Derumier <aderumier@odiso.com>