Fix bug in recreate-disks for DRBD instances

The new functionality in 2.4.2 for recreate-disks to change nodes is
broken for DRBD instances: it simply changes the nodes without caring
for the DRBD minors mapping, which will lead to conflicts in non-empty
clusters.

This patch changes Exec() method of this LU significantly, to both fix
the DRBD minor usage and make sure that we don't have partial
modification to the instance objects:

- the first half of the method makes all the checks and computes the
  needed configuration changes
- the second half then performs the configuration changes and
  recreates the disks

This way, instances will either be fully modified or not at all;
whether the disks are successfully recreate is another point, but at
least we'll have the configuration sane.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>