help: use global configuration if subcommand is help

Hi

i'm new user of @django, and i run `django-admin.py help check`, then my shell prints:
```
django.core.exceptions.ImproperlyConfigured: Requested setting DEFAULT_INDEX_TABLESPACE, 
but settings are not configured. You must either define the environment variable 
DJANGO_SETTINGS_MODULE or call settings.configure() before accessing settings.
```
I traced this problem is because i haven't configured my django settings file by `DJANGO_SETTINGS_MODULE`. but i don't think this should be set in help command, so i make this patch that use global configuration under `django-admin.py help [subcommand]`.