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.
@vuntz Fixed in https://github.com/toabctl/barclamp-crowbar/commit/2f52d16f0575b4c74dd8e3dc0902a364d8e8b242