Implement conversion from plain to drbd

This patch adds a new mode to instance modify, the changing of the disk
template. For now only plain to drbd conversion is supported, and the
new secondary node must be specified manually (no iallocator support).

The procedure for conversion works as follows:

- a completely new disk template is created, matching the count, size
  and mode of the instance's current disks
- we create manually (not via _CreateDisks) all the missing volumes
- we rename on the primary the LVs to the new name
- we create manually the DRBD devices

Failures during the creation of volumes will leave orphan volumes.
Failure during the rename might leave some disks renamed and some not,
leading to an inconsistent instance.

Once the disks are renamed, we update the instance information and wait
for resync. Any failures of the DRBD sync must be manually handled (like
a normal failure, e.g. by running replace-disks, etc.).

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