ganeti cluster backend is IPv4-only

I tried to reboot the ganeti cluster yesterday and it failed with:

root@fsn-node-04.torproject.org: Permission denied (publickey).

weasel diagnosed this as:

16:57:08 <weasel> etc/ssh/userkeys/root only has v4 addresses in from=..
16:57:22 <weasel> that is probably a bug.