Reintroduce the `fetch-ssh-keys` service so that GCE images that work
with NixOps can once again be built. Also, reformat the code a bit.
The service was removed in 88570538b3b19d60b00bc3905bbaaef17e5a5c94,
likely due to a comment saying it should be removed. It was still
needed for images to work with NixOps, however, and probably needed to
be replaced or rewritten rather than removed.