With some todos remaining, this patch switches the DRBD devices to use
the passed minors, and the cmdlib code (add instance and replace disks)
to request and assign minors to the DRBD disks.

Todos:
  - look at the disk RPC calls to see which can be optimized away, since
    we now know the minor beforehand
  - remove the _FindUnusedMinor usage from the few places it's still
    used (not for actual disks, but for temporary use in meta devs) and
    eventually replace with _CheckMinorUnused or such

Of course, this and/or the previous two patches break existing clusters.
Again.

Reviewed-by: imsnah