Move auto devops domain from ci/cd settings to cluster settings

Problem to solve

Currently, auto devops domain is configured on project's settings. This is problematic as it prevents users from setting up multiple domains for multiple clusters in an easy, intuitive way.

With multiple group clusters, we cannot set variables like AUTO_DEVOPS_DOMAIN per environment

Further details

Because each cluster has a unique ingress IP, each cluster should provide unique base domain configuration.

Solution

Domain

EE/CE
As an owner or maintainer, I am able to set a domain for both my group and project level clusters.

Group	Project

Auto DevOps project settings

EE/CE
As an owner or maintainer, I am no longer able to set a domain within my project Auto DevOps settings

Instance level domain

Instance level domain will not be changed in this issue and will be addressed when we add instance level clusters.

What does success look like, and how can we measure that?

(If no way to measure success, link to an issue that will implement a way to measure this)

Backend - To do list

1. Add domain field to Clusters::Cluster - https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/24368
   • Fixes Kubernetes Namespace migration https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/24536
2. Create migration to move information from ProjectAutoDevOps#domain to Clusters::Cluster#domain
3. UI modifications to the Cluster show view - https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/24580 && https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/24376
   • Make environment scope field a text_field:
   • Add "base domain" field to Cluster form
     • Display ingress IP + "nip.io" if available
   • Remove 'domain' field from Auto DevOps project settings
4. Inject KUBE_INGRESS_BASE_DOMAIN (along with AUTO_DEVOPS_DOMAIN) environment variable if present. https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/24580
5. Modifications to Auto-DevOps.gitlab-ci.yml script to include KUBE_INGRESS_BASE_DOMAIN https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/24741
6. Deprecate AUTO_DEVOPS_DOMAIN on 12.0 probably in a follow-up issue - https://gitlab.com/gitlab-org/gitlab-ce/issues/56959#note_135775342
7. Update documentation. https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/24740

Follow up issues

• 56715 - Clusters page: JS is disabled on render / https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/24583
• 56750 - Validation error appears twice on Cluster page
• 57357 - Technical UX issue on automating the help text for Domain field
• 56959 - Drop support for AUTO_DEVOPS_DOMAIN
• 57139 - Hide Kubernetes cluster warning if project has a cluster related

Links / references

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