Fix incomplete translation of "Update members" widget

Marks the default value for the search fields of the "Update members"
widget translatable. Also updates the English PO file accordingly.

Change-Id: I8c317de49087b4207bcb774dd241fd1e4591882b
Closes-Bug: #1239927

horizon/locale/en/LC_MESSAGES/django.po

@@ -58,6 +58,8 @@ msgstr ""
 
 #: tables/actions.py:381
 #: templates/horizon/common/_data_table_table_actions.html:13
+#: templates/horizon/common/_workflow_step_update_members.html:11
+#: templates/horizon/common/_workflow_step_update_members.html:17
 msgid "Filter"
 msgstr ""
 

horizon/templates/horizon/common/_workflow_step_update_members.html

@@ -8,13 +8,13 @@
     <div class="left">
       <div class="fake_table fake_table_header fake_{{ step.slug }}_table">
         <span class="members_title">{{ step.available_list_title }}</span>
-        <input type="text" name="available_{{ step.slug }}_filter" id="available_{{ step.slug }}" class="filter {{ step.slug }}_filter" placeholder="Filter">
+        <input type="text" name="available_{{ step.slug }}_filter" id="available_{{ step.slug }}" class="filter {{ step.slug }}_filter" placeholder="{% trans "Filter" %}">
       </div>
     </div>
     <div class="right">
       <div class="fake_table fake_table_header fake_{{ step.slug }}_table">
         <span class="members_title">{{ step.members_list_title }}</span>
-        <input type="text" name="{{ step.slug }}_members_filter" id="{{ step.slug }}_members" class="filter {{ step.slug }}_filter" placeholder="Filter">
+        <input type="text" name="{{ step.slug }}_members_filter" id="{{ step.slug }}_members" class="filter {{ step.slug }}_filter" placeholder="{% trans "Filter" %}">
       </div>
     </div>
   </div>