secret key

Maybe this fixes a stupid warning:

```
[Mon Dec 22 18:29:18.896822 2014] [:error] [pid 27207]   warn('SECRET_KEY is
set to an unsafe default. This should be set in local_settings.py for better
security')
[Mon Dec 22 18:29:19.107412 2014] [:error] [pid 27204]
/opt/graphite/webapp/graphite/settings.py:231: UserWarning: SECRET_KEY is set
to an unsafe default. This should be set in local_settings.py for better
security
```