Port replace disk/change node to the new DRBD RPCs

In replace disks to new secondary, since Attach (and therefore
call_blockdev_find) is not modifying the devices anymore, we need to
switch this LU to the new call_drbd_disconnect_net and
call_drbd_attach_net functions.

Due to the authentication needed in 2.0, we need to be more careful with
the activation order. In 1.2, we have the case that the new node was
directly activated with networking information, and could connect to the
primary while it was still connected or WFConnect to the old secondary.

In the new scheme, we:
  - create the new drbd in StandAlone mode
  - shutdown old secondary (primary becomes WFConnection)
  - disconnect primary (and thus it goes into StandAlone)
  - connect both primary and new secondary to network using the
    call_drbd_attach_net rpc

This should be safer, and is cleaner. This passes burnin.

Reviewed-by: ultrotter