gnt-group add: Fix diskparam fill

This was a pretty non-obvious bug. A cluster looks sane after
gnt-cluster init, however on a daemon restart the diskparameters had the
default filled in. The same applies to gnt-group add. This is due to the
nature that UpgradeConfig() from NodeGroups did just populate them with
defaults if something was set on it.

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