added predictable fixture data option

The comment on the config diff explains in what the option does: it allows for the creation of predictable data inside the development fixture (used by `rake db:seed_fu`) not generated by Faker which may be useful in interactive development tests.

The only currently generated data are users named `user0`, `user1`, etc. with password `aaaaaaaa` , which allow developers to easily login as multiple different users to see what the system looks like from their point of view.

The downside is that the data so generated be inadequate to end user demos or snapshots, which is why it is controlled by an option.

The option if off by default, so unless developers edit their `gitlab.yml`, nothing will change to their fixture data.

I did not edit the CHANGELOG since I am not sure if such a minor dev only change should be mentioned there.