add(image_set_group): New script to set the update group in prod images.

For generating images for groups other than the one given to build_image
run this script along with the usual image_to_vm.sh commands. To avoid
ambiguity with the 'latest' symlink, this script creates $group-latest
symlinks instead. build_image creates the new symlink too.
Merge pull request #245 from marineam/set-group

add(image_set_group): New script to set the update group in prod images.