Allow plain/DRBD conversions regardless of lack of disks

Allow plain/DRBD conversions regardless of lack of disks

Because of trivial issues, it was impossible to use standard mechanisms
to convert a diskless plain instance to DRBD and vice versa. This
patch fixes that with a mechanism which will work until later versions,
where instances without disks organically take on the diskless
template.

Signed-off-by: Hrvoje Ribicic <riba@google.com>
Reviewed-by: Helga Velroyen <helgav@google.com>