Extend gnt-os API to v20 - add support for osparams
Another osparam (img_ssh_key_url) and a hook to use it
Add a SELINUX relabel function for RHEL-like oses
A hook that randomizes the system crontab
Add support for remote image fetching.

Specify a URL (HTTP) and ganeti-instance-image will try to find the image
there before looking for a local file. Allows caching an image to the disk.
For this feature to work, sha1sum and curl are added as dependencies.