This url points to a json file that contains the value for other
OS parameters. Using this file we can pass parameters to snf-image
without writting them in Ganeti's config.data. This is
useful, for 2 reasons:
* Privacy: We don't want the password to appear in the Ganeti config
* Speed: Some parameters like IMG_PERSONALITY can be quite large (a few
KB) and since ganeti shares the instance configuration among different
nodes, the overall system will start getting slower if many instances
with IMG_PERSONALITY exist.