Fix editing Tags for Cloud Subnets

Fixes https://bugzilla.redhat.com/show_bug.cgi?id=1534915

Fix editing Tags for Cloud Subnets when going to Networks -> Networks,
choosing a network, selecting Cloud Subnets and selecting some Subnets
by checking the checkboxes in the list and clicking on Edit Tags under
Policy.

app/controllers/cloud_network_controller.rb

@@ -30,6 +30,8 @@ def button
       javascript_redirect :action => "edit", :id => checked_item_id
     when "cloud_network_new"
       javascript_redirect :action => "new"
+    when "cloud_subnet_tag"
+      return tag("CloudSubnet")
     when "custom_button"
       custom_buttons
       return