Fixed #12976 -- Fixed the sequence reset commands issued by sqlflush in a...

Fixed #12976 -- Fixed the sequence reset commands issued by sqlflush in a multidb setup. To achieve this, database introspection was modified to utilize routing information to determine if a model should be available. Thanks to pczapla for the report.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@12753 bcc190cf-cafb-0310-a4f2-bffc1f526a37