Set CSS dir for the 404 error page

* The current error page that comes up does not correctly link to the CSS
files. For example, see the source on
http://docs.openstack.org/kilo/networking-guide/scenario_legacy_lb.html.

Change-Id: I33aff65da9be156b0c05139fe96a045e660a203d

www/errorpage.html

@@ -7,6 +7,7 @@
     <meta name="viewport" content="width=device-width, initial-scale=1">
     <title>404 Not Found</title>
 <!-- CSS in: {{ cssdir }}-->
+{% set cssdir = './common/css/' %}
 {% include 'templates/css.tmpl' %}
 {% block content %}
 <!-- Begin Page Content -->