No, you were right the first time :-) At least if we're talking about a Debian or Ubuntu-based host.
You should not touch the files under /usr/share/ganeti/os/ or /etc/default/ganeti-instance-deboostrap/
You should edit /etc/ganeti/instance-debootstrap/variants.list and /etc/ganeti/instance-debootstrap/variants/<foo>.conf
where <foo> is the variant you are making. e.g.
==> /etc/ganeti/instance-debootstrap/variants.list <==
default
precise
==> /etc/ganeti/instance-debootstrap/variants/precise.conf <==
SUITE="precise"
EXTRA_PKGS="acpid,console-tools,udev,linux-image-virtual,openssh-server"
COMPONENTS="main,restricted,universe,multiverse"
ARCH="amd64"
You should also find that /usr/share/ganeti/os/debootstrap/variants.list is a symlink to /etc/ganeti/instance-debootstrap/variants.list
If that's not true, then you may have a broken instance of the ganeti-instance-debootstrap package. I use version 0.14 from wheezy-backports.
HTH,
Brian.