Check that at least 2 disks are selected for raid

When configuring RAID, 2 disks are at least needed. Otherwise i.e. the
autoyast.xml template gets wrong values and the installation aborts.
> Btw, I'm not sure the error should live in nodes.list. This sounds wrong.

And what's the correct place? Just "error.raid_disks_error" ? Or "nodes.edit.raid_disks_error" ?