gitlab.rb.template doesnt contain `gitlab_rails['enable'] = true`

Summary

Our gitlab.rb.template file should illustrate how to disable rails, as this may be desired for specialized-role deployments (i.e. postgres node, redis master, etc).

It may be the case that defining a role disables rails if appropriate (and our docs certainly seem to say that's the case).

However, our chef cookbook does currently have logic that will include or exclude gitlab-shell and gitlab-rails based on the presence of gitlab_rails['enable'] = bool in gitlab.rb and our template should reflect that.