slapos_jio: Support list of elements expressed with oneOf

By following feature of JSON Schema Draft-06 render a structure like:

  "element": {
    "oneOf": [
      {
        "const": "value1",
        "title": "Description1"
      },
      {
        "const": "value2",
        "title": "Description2"
      }
    ]
  }

To:

  <select size="1">
    <option value=""></option>
    <option value="value1">Description1</option>
    <option value="value2">Description2</option>
  </select>

JSON Schema types array and object are stringified to be safely represented
as option keys.

master/bt5/slapos_jio/PathTemplateItem/web_page_module/rjs_gadget_erp5_page_slap_parameter_form_js.js

@@ -122,6 +122,37 @@
     return input;
   }
 
+  function render_selection_oneof(json_field, default_value) {
+    var input = document.createElement("select"),
+      option = document.createElement("option"),
+      optionz;
+    input.size = 1;
+    option.value = "";
+    if (default_value === undefined) {
+      option.selected = "selected";
+    }
+    input.appendChild(option);
+    json_field.oneOf.forEach(function (element, index) {
+      if ((element['const'] !== undefined) && (element.title !== undefined)) {
+        var value;
+        if ((json_field.type == 'array') || (json_field.type == 'object')) {
+          // Support for unusual types
+          value = JSON.stringify(element['const']);
+        } else {
+          value = element['const'];
+        }
+        optionz = document.createElement("option");
+        optionz.value = value;
+        optionz.textContent = element.title;
+        if (value === default_value) {
+          optionz.selected = "selected";
+        }
+        input.appendChild(optionz);
+      }
+    });
+    return input;
+  }
+
   function render_textarea(json_field, default_value, data_format) {
     var input = document.createElement("textarea");
     if (default_value !== undefined) {
@@ -141,6 +172,10 @@
       return render_selection(json_field, default_value);
     }
 
+    if (json_field.oneOf !== undefined) {
+      return render_selection_oneof(json_field, default_value);
+    }
+
     if (json_field.type === "boolean") {
       json_field['enum'] = [true, false];
       if (default_value === "true") {