Fixed #10487 -- Refactored the database-backend-selection logic into a...

Fixed #10487 -- Refactored the database-backend-selection logic into a function, django.db.load_backend. Thanks to Alex Gaynor for the initial patch

git-svn-id: http://code.djangoproject.com/svn/django/trunk@10045 bcc190cf-cafb-0310-a4f2-bffc1f526a37