title: formly Default value in dynamic options
body: <p>I use angular js formly to create my web forms.
  my problem is when is use select with dynamic options the defaultValue property dos not work. but when is set static data to options it is work.
  how can i set default value (selected option) in formly when i get options from remote server ?</p>
  
  <pre><code>{
  className: 'col-sm-3',
  key: 'cityId',
  type: 'select',
  templateOptions: {
      placeholder: 'City',
      required: true,
      label: 'City',
      valueProp: 'id',
      labelProp: 'name',
      options: $scope.cityCombo
  },
  
  defaultValue: '3209C692-B8D4-4AB0-9F40-008A7C4644F9',
  </code></pre>
  
  <p>}</p>