Less visual steps
/cc @dzaporozhets @jacobvosmaer
This adds the template step from https://gitlab.com/gitlab-org/omnibus-gitlab/issues/158
It gives a hint how people should configure postfix (something I was insecure about years ago)
It removes the code block around the credentials (you don't want people to think it is code, makes it visually look like less steps, current look can be improved)