Fix default Site ID to use settings.SITE_ID

[sebasmagri]

This fixes syncdb when using mongodb-engine as the default SITE_ID=1 is
not a valid ObjectID in MongoDB. Setting a valid SITE_ID is already
suggested by mongodb-engine, but it does not have any effect if the
SITE_ID is hardcoded for the default site.

[aburgel]

We're trying to have as few differences as possible to our fork of django. This will make it easier to keep things in sync and eventually get rid of the fork altogether. Is there another way to handle this? Or if not, have you looked into contributing it to the upstream django project?

[wrwrwr]

As of ticket #23945 this is in Django (master/1.8). I guess, we could backport it for nonrel.

PS: MongoDB itself seems to support non-ObjectID keys, so in theory the backend could allow them. For reference: the error message was introduced in django-nonrel/mongodb-engine@af136ba, but the ObjectId cast was there a lot earlier.