South migration uses get_auth_model

Using get_auth_model gives south the _currently_ configured AUTH_USER_MODEL. This works fine for new initial database migrations, but it will fail when AUTH_USER_MODEL changes over the lifetime of a django DB.

So it is not a perfect solution, but it works in more situations that the current code which does use get_auth_model for the model definition, but not for the south migration.

Related south issue: http://south.aeracode.org/ticket/1179