hosts: nixos: add host-specific secrets module

This is the same logic as the common module, but for secrets that don't
need to be shared to different hosts.

hosts/nixos/aramis/secrets/secrets.nix

@@ -0,0 +1,13 @@
+# Host-specific secrets
+let
+  keys = import ../../../../keys;
+
+  all = [
+    # This host is a laptop, it does not have a host key
+    # Allow me to modify the secrets anywhere
+    keys.users.ambroisie
+  ];
+in
+{
+  # Add secrets here
+}