Specify ssh-keys as list of strings, rather than having a single string.

Older versions of nixpkgs allowed you to specify multiple keys by having
newline separated keys in your string. However, this worked essentially by
accident, and is now explictly disallowed.

I noticed this because I had configured multiple keys for the local grid.  This
isn't currently impacting my ability to work, but it seems like a worthwhile
improvement anyway. This will be necessary (for example) if/when multiple people are
given root access to our storage nodes.