Fix disk replace secondary with static minors

The code in 'updating instance configuration' section of the replace
disks with change secondary node was setting a wrong new logical_id for
the drbd devices (only set the new node, not the new minor). The patch
fixes this by remembering the new logical_id that we use for activating
the drbd and reusing it (instead of recomputing).

Since the minors are not needed to be allocated before the LVs are
created, we also move the minor allocation step (currently as None)
after this step.

Reviewed-by: imsnah