Login should be required to use the Munin Master Admin interface

This pull request enable authentication for the munin master admin interface. It require also some changes for the mibe image we build, because the admin page will be not available on `/admin/` by default. This should be handled by `nginx`. I will modify the mibe image later.

Required for mibe image:

```
settings.py:

LOGIN_REDIRECT_URL = '/admin'
```

```
nginx.conf:

proxy_set_header SCRIPT_NAME '/admin';
```