fix the selected index disappearing from the select box

The reason this was happening was that whenever the indices variable was
updated the whole select and ng-options values were updated and for some
reason when that happened it ignores the model you have selected. So if
you had a 3s autorefresh it would take 3s to disappear, or 5s or 1min...
The way this fix works is by adding a default value to the ng-options.

Really this is just a visual fix because the ng-model field_index was never
actually reset, you could still select from the other sub-menus.

However, maybe a better approach would be not to auto-refresh the index list
and instead provide a button to carry out that action explicitly because
auto-updating also causes some trouble when you have many indices and you
select form the list since it keeps auto-updating it brings you to the top every 3s
(or whatever your interval is)