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/297a0c965ac5f408cf63613af06c9c7a37209ab6