Fix for multiple VGs - PlainToDrbd and replace-disks

Converting an instance from 'plain' to 'drbd'.  The old code would
create the drbd volumes in the default VG and then the renames would
fail.  This fix pulls the plain VG names from the existing volumes and
places it into the new disk template.

Running 'replace-disks' has a similar issue with the new disks going
into the wrong VG and then the rename failing.

Their might be a similar issue with 'recreate-disks', but I actually
have no idea what recreate-disks does, so did not look into it.

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