Disable password login via SSH in apex

Reviewed-by: Aleix Boné <abonerib@bsc.es>

m/apex/configuration.nix

@@ -57,6 +57,11 @@
     };
   };
 
+  services.fail2ban.enable = true;
+
+  # Disable SSH login with password, allow only keypair
+  services.openssh.settings.PasswordAuthentication = false;
+
   networking.firewall = {
     extraCommands = ''
       # Blackhole BSC vulnerability scanner (OpenVAS) as it is spamming our