Forward-port the ganeti 2.0 cfgupgrade

2.1's cfgupgrade doesn't support upgrading from 1.2, only from 2.0.
However, it's trivial to forward-port the 2.0 cfgupgrade to work with
2.1, thus providing an upgrade path for 1.2 users directly to 2.1,
without the intermediate step of installing 2.0.

The new cfgupgrade12 file has just a few trivial changes from 2.0, and
skipping the load of the config file.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: René Nussbaumer <rn@google.com>