Make GitLab CI easier to setup

The process now consists of three steps https://gitlab.com/gitlab-org/omnibus-gitlab/blob/master/doc/gitlab-ci/README.md#getting-started

1. Add url
2. Run reconfigure
3. Generate app id and secret in the web interface
4. Add them to gitlab.rb
5. Run reconfigure again

We want to make using GitLab CI as easy as possible. Can we reduce the number of steps here?

Maybe we can generate an app-id and secret when /etc/gitlab/gitlab.rb is initially written?

Asking @jacobvosmaer and @marin