Resolve "Provide zero-configuration domain name for Auto DevOps using nip.io"

See EE counterpart MR

What does this MR do?

Default AUTO_DEVOPS_DOMAIN variable to <ingress-ip>.nip.io.

• Added DeploymentPlatform#find_cluster so that EE can extend this method
• Refactors DeploymentPlatform to use find_cluster
• Moves AUTO_DEVOPS_DOMAIN to project#auto_devops_domain_variable, which is then used in CI::Build#persisted_environment_variables

This follows the following rules of precedence :

1. cluster(s) IP based domain
2. Auto DevOps domain or instance domain
3. CI scoped variables (Settings > CI/CD > Variables, AUTO_DEVOPS_DOMAIN, with any environment scope)

NOTE : The move to deployment_variables basically means that AUTO_DEVOPS_DOMAIN variable will now only be available for the jobs that sets a environment in the gitlab-ci.yaml file.

Are there points in the code the reviewer needs to double check?

Security - how can we trust and verify the value of the external_ip given to GitLab by any ingress status

Any subtle interactions with multiple clusters, if we provide zero-config for this case

Why was this MR needed?

Screenshots (if relevant)

No Kubernetes cluster setup:

With Kubernetes cluster setup:

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• [-] API support added
• Tests added for this feature/bug
• Conforms to the code review guidelines
  • Has been reviewed by a UX Designer
  • [-] Has been reviewed by a Frontend maintainer
  • Has been reviewed by a Backend maintainer
  • [-] Has been reviewed by a Database specialist
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• If you have multiple commits, please combine them into a few logically organized commits by squashing them
• Internationalization required/considered
• End-to-end tests pass (package-and-qa manual pipeline job)

To Do Backend

• Get the ingress IP from the the only cluster
• For multiple clusters
  • Get the ingress IP from the Ingress for the * cluster for the project;
  • [-] Provide ingress IP for other clusters so that we can assemble AUTO_DEVOPS_DOMAIN scoped to the environments
• Create a domain string by using the IP + 'nip.io'. Use that default domain string for AUTO_DEVOPS_DOMAIN if the ProjectAutoDevops does not have a domain or instance domain set by the user.
• Update the help text (see also https://gitlab.com/gitlab-org/gitlab-ce/issues/45561)
• [-] Warn if we cannot use the ingress ? (e.g. ingress address is not IP)

What are the relevant issue numbers?

Closes #45560 (moved)