Generate cluster resources

What does this MR do and why?

Generate cluster resources.

Add a new class under the prerequisite, that will be executed just before the CI pipeline starts. #complete! method calls KAS three (or four) times to complete the process. (1) Get the template, (2) Render the template, and (3) Apply the template.

It does NOT support:

• resource_management.enabled flag. This will be supported in the follow-up MR.
• Detailed error information. (Follow-up issue: Better error information for GitLab-managed Kub... (#513694))
• Custom template file name. (Will be supported in the future)

References

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• Part of #507268 (closed)

MR acceptance checklist

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Before	After

How to set up and validate locally

1. Run GDK. Stop KAS. (gdk start && gdk stop gitlab-k8s-agent)
2. Run KAS with the latest master branch in your local. (See Running Kas and agentk locally)
3. Set up a example project, and connect it to the gitlab agent. Add agent config file and allow ci_access to the agent.(Authorize the agent to access your projects) For example:

ci_access:
  projects:
    - id: path/to/project

4. Add CI job .gitlab-ci.yml

deploy:
  image:
    name: bitnami/kubectl:latest
    entrypoint: ['']
  environment:
    name: env1 # Change to your environment name
    deployment_tier: staging
  script:
    - kubectl config get-contexts
    - kubectl get ns

4. Modify resource_management_enabled? method to return true to verify this prerequisite works correctly. Leave it as-is so that you can make sure it won't work and nothing changes from current behavior.
5. Push commit and run CI pipeline.
6. If resource_management_enabled? method is updated to return true, it will execute the newly added prerequisite class. That will communicate with KAS and apply environment template. The default template will create the namespace (e.g. env1-24-5) and role binding. If resource_management_enabled? method returns false, it does nothing.