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.
> This message is slightly misguiding (and group_error also is, in the context of save_node, which is why I suggested a new message): we don't just fail to save specific settings, but everything.

@vuntz: What about https://github.com/toabctl/barclamp-crowbar/commit/f22bf0f09e935a1db6eff6d3fdd208a62e30e9fe and https://github.com/toabctl/barclamp-crowbar/commit/8faea0f690c1327992de62abae70b4ffe1fc3cb9 ?