nixos/waagent: specify OS.OpensslPath by default

nixos/modules/virtualisation/waagent.nix

@@ -192,6 +192,15 @@ let
             If set to `null`, the system defaults are used.
           '';
         };
+
+        OpensslPath = lib.mkOption {
+          type = types.nullOr types.path;
+          default = lib.getExe pkgs.openssl;
+          defaultText = literalExpression "lib.getExe pkgs.openssl";
+          description = ''
+            Specify a path for the openssl binary to use for cryptographic operations.
+          '';
+        };
       };
 
       HttpProxy = {