[1.3.X] Fixed #16353 -- don't try to create Site objects on all databases....

[1.3.X] Fixed #16353 -- don't try to create Site objects on all databases. Refs #15573, #15346. Thanks Aymeric Augustin for the report and the patch.

Backport of r16868 in trunk.

git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.3.X@16869 bcc190cf-cafb-0310-a4f2-bffc1f526a37