Extend Ganeti OS Interface to v20

These 2 patches extend `ganeti-instance-image`'s OS API to v20 and add support for basic osparams.

The first patch adds `img_format` and `img_id`. This allows specifing the os of the image during instance creation. These parameters are required. The original commit were made by `Constantinos Venetsanopoulos <cven@cslab.ece.ntua.gr>`  on 05-2011. The verify script is new.

The second commit adds support for a second osparam (`img_ssh_key_url`) and a hook that injects it into the image (for the root user). This was made by `Apollon Oikonomopoulos <apollon@noc.grnet.gr>` on 06-2011. The img_ssh_key_url osparam is not required.