Change config upgrade to be explicit

Currently the config upgrade is done at each object instantiation, that
means that ganeti-noded will run UpgradeConfig on all objects received
remotely (instances, disks, nics). This is not so good, so this patch
changes it so that only the ConfigWriter runs this method at
configuration load time.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>