nixos/graphite-service: fix startup

I couldn't get graphite to start. This fixes it.

CC @offlinehacker.

I have another issue with graphite. It has little to do with this pull requrest, except that graphite may need some more fixes :-)

Disclaimer: I'm a graphite newbie.

When I access http://localhost:8080 I get a white page with a black bar on top with the text "Graphite" on the left and "login, documentation [...]" on the right. I don't know whether that's correct or not (I haven't really configured/customized graphite yet), but the log seems to indicate issues:

```
waitress-serve[25346]: Could not import graphite.local_settings, using defaults!
waitress-serve[25346]: /nix/store/sdfg78hg15qizyh8ws5z55zamxhrsjns-python2.7-graphite-web-0.9.12/lib/python2.7/site-packages/graphite/settings.py:231: UserWarning: SECRET_KEY is set to an unsafe default. This should be se... for better security
waitress-serve[25346]: warn('SECRET_KEY is set to an unsafe default. This should be set in local_settings.py for better security')
waitress-serve[25346]: ERROR:django.request:Internal Server Error: /content/img/carbon-fiber.png
waitress-serve[25346]: Traceback (most recent call last):
waitress-serve[25346]: File "/nix/store/2l86g94bvgd42p0w657gxzz212rj9kqs-python2.7-Django-1.3.7/lib/python2.7/site-packages/django/core/handlers/base.py", line 150, in get_response
waitress-serve[25346]: response = callback(request, **param_dict)
waitress-serve[25346]: File "/nix/store/2l86g94bvgd42p0w657gxzz212rj9kqs-python2.7-Django-1.3.7/lib/python2.7/site-packages/django/utils/decorators.py", line 93, in _wrapped_view
waitress-serve[25346]: response = view_func(request, *args, **kwargs)
waitress-serve[25346]: File "/nix/store/2l86g94bvgd42p0w657gxzz212rj9kqs-python2.7-Django-1.3.7/lib/python2.7/site-packages/django/views/defaults.py", line 18, in page_not_found
waitress-serve[25346]: t = loader.get_template(template_name) # You need to create a 404.html template.
waitress-serve[25346]: File "/nix/store/2l86g94bvgd42p0w657gxzz212rj9kqs-python2.7-Django-1.3.7/lib/python2.7/site-packages/django/template/loader.py", line 157, in get_template
waitress-serve[25346]: template, origin = find_template(template_name)
waitress-serve[25346]: File "/nix/store/2l86g94bvgd42p0w657gxzz212rj9kqs-python2.7-Django-1.3.7/lib/python2.7/site-packages/django/template/loader.py", line 138, in find_template
waitress-serve[25346]: raise TemplateDoesNotExist(name)
waitress-serve[25346]: TemplateDoesNotExist: 404.html
waitress-serve[25346]: ERROR:django.request:Internal Server Error: /content/js/ext/adapter/ext/ext-base.js
waitress-serve[25346]: Traceback (most recent call last):
```

My graphite/statsd config is

```
    statsd = {
      enable = true;
      extraConfig = ''
      '';
    };

    graphite = {
      web.enable = true;
      web.port = 8080;
      carbon.enableCache = true;
      carbon.storageSchemas = ''
        [server_load]
        priority = 100
        pattern = ^servers\.
        retentions = 60:43200,900:350400
      '';
    };
```