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

What does this MR do?

Adds a node gitlab_rails['enable'] = true to the gitlab.rb template illustrating that it's possible to disable gitlab-rails (and implicitly, gitlab-shell).

Slack context while it lasts: https://gitlab.slack.com/archives/C1FCTU4BE/p1615416574031500

Related issues

#6030

Checklist

See Definition of done.

For anything in this list which will not be completed, please provide a reason in the MR discussion

Required

• Merge Request Title, and Description are up to date, accurate, and descriptive
• MR targeting the appropriate branch
• MR has a green pipeline on GitLab.com
• Pipeline is green on dev.gitlab.org if the change is touching anything besides documentation or internal cookbooks
• trigger-package has a green pipeline running against latest commit

Expected (please provide an explanation if not completing)

• Test plan indicating conditions for success has been posted and passes
• Documentation created/updated
• Tests added
• Integration tests added to GitLab QA
• Equivalent MR/issue for the GitLab Chart opened

Closes #6030