use by_organization_user view to determine org membership

## Known Issues ##

* We can use the orgname -> org_id cache [here](https://github.com/opscode/chef_db/blob/master/src/chef_db.erl#L230) to fetch the org_id
* This is untested. I only ran this code on my branch which had `chef_db:is_user_in_org` stubbed to `true`