Add some documentation to host-to-key-validation

I plan to add some other metadata for storing the client and host keys of each
host, which would allow automatic-updating of the known_hosts and
authorized_keys files if using the legacy (non-hesobase-based) SSH system.
Add missing } in README.md
Add feature to automatically add lines to known_hosts and authorized_keys

Once polished, this should make configuration much easier.
Do not lock the .ssh key files when editing them
Remove unused imports in key_files.clj