do not partition disks in iSCSI (team#40775)

We decided against repartitionning the disk at all. To quote the above
ticket:

> the rationale, for the SAN, is especially that we have a lot of
> devices already for the LUNs. a single LUN creates six devices in
> the kernel (3 paths for each controller, so six), which makes
> commands like lsblk and multipath -ll really unreadable quick. by
> having three LUNs per VM, we explode this to eighteen (18!) devices
> per VM, which is really a mess. i even wonder if we might hit the
> number of maximum devices allowed internally...
>
> point is: one LUN per partition per VM is messy. and anyways, it's
> not how "the cloud" works, typically; when you ask for a 100GB VM
> from Amazon, they don't give you the VM partitionned in whatever
> they think is the right chunk, they just give you a f*kn 100GB disk,
> period. in our case, ganeti-instance-debootstrap hacks at this to
> prepend a MBR (and with GPT, we should probably have an EFI
> partition), but whatever: point is it's just one fkn disk.