nix/base: add remoteMachines target that can be realised on a linux build host

e.g.:

% nix-realise-remote --arg networkExprs ./ie.nix '<upcast/eval-deployment.nix>' -A remoteMachines
warning: you did not specify `--add-root'; the result might be removed by the garbage collector
/nix/store/m1kmmpjacjqzfbbwc1jf11inll7zxjpb-upcast-machines-remote
Shared connection to hydra closed.
% ssh hydra cat /nix/store/m1kmmpjacjqzfbbwc1jf11inll7zxjpb-upcast-machines-remote
{"node1":"/nix/store/n0cgpvf9wh7qfrqg1issp9v7xswaqclz-nixos-14.10pre-git","node2":"/nix/store/n0cgpvf9wh7qfrqg1issp9v7xswaqclz-nixos-14.10pre-git"}