Support setting allowed_hosts in gitlab.yml

What does this MR do?

Support setting allowed_hosts in gitlab.yml

Testing

1. Set a value for global.appConfig.allowedHosts
2. Deploy
3. In the webservice container, open up a Rails console and check the value of Rails.application.config.hosts.
4. Ensure that it has localhost, the IP of the pod (you can find this by doing kubectl get pods -o wide), and the DNS entry of the webservice deployment.

Related issues

Closes #4110

Author checklist

For general guidance, please follow our Contributing guide.

Required

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

• Merge Request Title and Description are up to date, accurate, and descriptive.
• MR targeting the appropriate branch.
• MR has a green pipeline.
• Documentation created/updated.
• Tests added/updated, and test plan for scenarios not covered by automated tests.
• Equivalent MR/issue for omnibus-gitlab opened.

Reviewers checklist

• MR has a green pipeline on https://gitlab.com/gitlab-org/charts/gitlab.
• Consider downstream impact to the Operator, as per evaluating impact from changes to GitLab Chart.