Provide help-text capability for server-side filter choices

We have server-side filtering available for Project/Admin Instances,
and we have more to come (see bp). Some of these filters should provide
help text for what format user needs to input to filter.
For example, 'changed-since' filter is very vague. Need to provide
date format to user.

This patch adds a tooltip to the select option upon hovering.

How to use: add help text to the filter_choice in
/dashboards/admin/instances/table.py.

filter_choices = (
('name', _("Name ="), True),
('date_created', _("Date Created="), True, _("ISO 8601 format")),
...)

Change-Id: I436a3de51aa8182200cf56ad5502a85d1cd1b11c
Partially-implements: blueprint server-side-filtering

horizon/tables/actions.py

@@ -528,10 +528,11 @@ class FilterAction(BaseAction):
         return False
 
     def get_select_options(self):
-        """Provide the value and string for the template to render.
+        """Provide the value, string, and help_text (if applicable)
+        for the template to render.
         """
         if self.filter_choices:
-            return [x[:2] for x in self.filter_choices]
+            return [x[:4] for x in self.filter_choices]
 
 
 class NameFilterAction(FilterAction):

horizon/templates/horizon/common/fields/_themable_select.html

@@ -23,8 +23,10 @@
   </button>
   <ul class="dropdown-menu">
     {% for option in options %}
-      <li data-original-index="{{ forloop.counter0 }}">
-        <a data-select-value="{{ option.0 }}"
+      <li data-original-index="{{ forloop.counter0 }}"
+        data-toggle="tooltip" data-placement="top"
+        {% if option.3 %} title="{{ option.3 }}" {% endif %}>
+          <a data-select-value="{{ option.0 }}"
           {% if option.2 %}
             {{ option.2|safe|default:'' }}
           {% endif %}>{{ option.1 }}</a>