check required on add

previsouly was checked on key down, but that does the check too soon.
Next attempt was on blur, but that had numerous problems.  This now checkes when the add button is clicked.
works for entity_select widget, too
Checks upon form submission

https://fedorahosted.org/freeipa/ticket/1437

install/ui/service.js

@@ -110,6 +110,7 @@ IPA.service_add_dialog = function(spec) {
     var that = IPA.add_dialog(spec).
         field(IPA.widget({
             name: 'krbprincipalname',
+            optional:true,
             hidden: true
         })).
         field(IPA.service_select_widget({