Add internal and external URL config for KAS

What does this MR do?

Adds auto-configuration of the fields gitlab_kas.enabled, gitlab_kas.internal_url and gitlab_kas.external_url for gitlab-rails, so that the GitLab backend knows if KAS is configured and where to find it.

Related issues

gitlab#299850 (closed)

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

Test plan

I ran a quick smoke test as a sanity check in addition to the new specs.

Setup

Ran docker image from the latest QA pipeline for this MR:

docker run \
    -v $(pwd)/files/gitlab-cookbooks/gitlab-kas:/opt/gitlab/embedded/cookbooks/gitlab-kas \
    --rm -it \
    registry.gitlab.com/gitlab-org/build/omnibus-gitlab-mirror/gitlab-ee:13.10.0-rfbranch.261417095.e7c14263-0 bash

Enabled kas and set one custom URL value

echo "gitlab_kas['enable'] = true" > /etc/gitlab/gitlab.rb
echo "gitlab_rails['gitlab_kas_internal_url'] = 'abcd.example.com'" >> /etc/gitlab/gitlab.rb

Configured gitlab:

/opt/gitlab/embedded/bin/runsvdir-start &
gitlab-ctl reconfigure

Expectations and results

1. /var/opt/gitlab/gitlab-kas/gitlab-kas-config.yml should show the new customizations ✅
2. The custom value for gitlab_kas_internal_url should show under gitlab_kas.internal_url in /var/opt/gitlab/gitlab-rails/etc/gitlab.yml ✅