Fix UpgradeConfig of Disk object regards disk params

This bug was found during disk parameter debugging. While looking at the
config some values seem present on the disk parameters, but that's not
expected behaviour. This patch fixes this, and also fixes the "broken"
configs automatically upon masterd restart.

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