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>