Fix draw_graph_div for display_status=True (e.g. statsd plugin).

Stock configuration with statsd plugin enabled breaks the webconsole (empty page after connect):

```
Traceback (most recent call last):
  File "/home/marcus/semantics/projects/graunt/graunt-freimann/target/lib/python2.7/site-packages/circusweb/circushttpd.py", line 122, in render_template
    return template.generate(**namespace)
  File "/home/marcus/semantics/projects/graunt/graunt-freimann/target/lib/python2.7/site-packages/mako/template.py", line 443, in render
    return runtime._render(self, self.callable_, args, data)
  File "/home/marcus/semantics/projects/graunt/graunt-freimann/target/lib/python2.7/site-packages/mako/runtime.py", line 807, in _render
    **_kwargs_for_callable(callable_, data))
  File "/home/marcus/semantics/projects/graunt/graunt-freimann/target/lib/python2.7/site-packages/mako/runtime.py", line 839, in _render_context
    _exec_template(inherit, lclcontext, args=args, kwargs=kwargs)
  File "/home/marcus/semantics/projects/graunt/graunt-freimann/target/lib/python2.7/site-packages/mako/runtime.py", line 865, in _exec_template
    callable_(context, *args, **kwargs)
  File "/home/marcus/semantics/projects/graunt/graunt-freimann/target/lib/python2.7/site-packages/circusweb/templates/base.html", line 109, in render_body
    ${self.body()}
  File "/home/marcus/semantics/projects/graunt/graunt-freimann/target/lib/python2.7/site-packages/circusweb/templates/index.html", line 105, in render_body
    ${draw_graph_div(plugin, plugin.split(':')[1].replace('-', '.'), True, endpoint=stat_endpoint)}
  File "/home/marcus/semantics/projects/graunt/graunt-freimann/target/lib/python2.7/site-packages/circusweb/templates/index.html", line 1, in draw_graph_div
    <%inherit file="base.html"/>
  File "/home/marcus/semantics/projects/graunt/graunt-freimann/target/lib/python2.7/site-packages/circusweb/templates/index.html", line 68, in render_draw_graph_div
    <div style="width: 20px; float: right; margin-right: 10px;"><a class="${controller.get_status(watcher, endpoint)}" title="${controller.get_status(watcher, endpoint)}" href="${reverse_url('switch_status', endpoint, name=watcher)}"></a></div>
  File "/home/marcus/semantics/projects/graunt/graunt-freimann/target/lib/python2.7/site-packages/circusweb/controller.py", line 129, in get_status
    res = client.send_message('status', name=name)
AttributeError: 'NoneType' object has no attribute 'send_message'
```

Reason is that you got yourself confused about endpoint vs stat_endpoint.  It's a bit unfortunate that the outer loop uses "endpoint" for what is actually the stat_endpoint, while the controller.py get_status call use endpoint for what it is.  I cleaned up draw_graph_div, which is sufficient to make it work.  For clarity, you might want to rename endpoint in the outer loop to stat_endpoint, too.