Fixed add_bootstrap_class template filter

Made sure that the add_bootstrap_class filter correctly obtains the
widget type to prevent it applying the form-control css class onto
parts of forms that shouldn't have it.

Change-Id: I307780b241e70de38e405cbffc4a35859b9aaf1b
Closes-Bug: 1367302

horizon/templatetags/form_helpers.py

@@ -22,11 +22,11 @@ def add_bootstrap_class(field):
 
     This is so that Bootstrap styles it properly.
     """
-    if not isinstance(field, (
-        django.forms.CheckboxInput,
-        django.forms.CheckboxSelectMultiple,
-        django.forms.RadioSelect,
-        django.forms.FileInput,
+    if not isinstance(field.field.widget, (
+        django.forms.widgets.CheckboxInput,
+        django.forms.widgets.CheckboxSelectMultiple,
+        django.forms.widgets.RadioSelect,
+        django.forms.widgets.FileInput,
         str,
     )):
         field_classes = set(field.field.widget.attrs.get('class', '').split())