rename sshPrivateKeyFile also in other files

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.

This is a follow-up to !166 (c9ddc8ad), where I missed one occurence.

- The serivice can be reached by other machines.
- The metrics cannot be reached by other machines.