Relax replace_disks_all meaning for drbd8

In order to make the replace secondary action to be done via the same
opcode parameters for both remote_raid1 and drbd, we must allow the
LUReplaceDisks to change replace_disks_all for drbd with non-empty
remote_node into replace_disks_sec.

This fixes gnt-node evacuate and also allows gnt-instace replace-disks
to have the same syntax for both remote_raid1 and drbd (when replacing
the secondary).

Reviewed-by: ultrotter