ssh, ssh-for-each, info (possibly others) broken with memory backend

[pennae]

when using the memory backend multiple subcommands of nixops either misbehave or fail entirely while the "important" stuff like deploy works as expected:

• nixops info lists all machines of a network, but reports them as missing
• nixops ssh reports "unknown machine" for every machine listed by info
• nixops ssh-for-each doesn't do anything at all

tried to get to the bottom of it but got nowhere.