Add FF to toggle cert-based clusters features

We're deprecating cert-based clusters and all of its related features
will be removed. So we've decided to use feature flags to hide the
features in the major milestone instead of right away removing them.
This MR introduces the first usage of this FF, but other MRs will come
to put this FF in other places.

At this time, we're hiding everything that makes use of
deployment_platform. The most impactful thing is not creating rollout
jobs anymore during Auto DevOps pipelines. So we're also adding tests
for this.

app/models/concerns/deployment_platform.rb

@@ -3,6 +3,8 @@
 module DeploymentPlatform
   # rubocop:disable Gitlab/ModuleWithInstanceVariables
   def deployment_platform(environment: nil)
+    return if Feature.disabled?(:certificate_based_clusters, default_enabled: :yaml, type: :ops)
+
     @deployment_platform ||= {}
 
     @deployment_platform[environment] ||= find_deployment_platform(environment)

config/feature_flags/ops/certificate_based_clusters.yml

+---
+name: certificate_based_clusters
+introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/81054
+rollout_issue_url: https://gitlab.com/gitlab-org/gitlab/-/issues/353410
+milestone: '14.9'
+type: ops
+group: group::configure
+default_enabled: true

spec/lib/gitlab/ci/templates/auto_devops_gitlab_ci_yaml_spec.rb

@@ -238,14 +238,34 @@ RSpec.describe 'Auto-DevOps.gitlab-ci.yml' do
         end
 
         it_behaves_like 'pipeline with Kubernetes jobs'
+
+        context 'when certificate_based_clusters FF is disabled' do
+          before do
+            stub_feature_flags(certificate_based_clusters: false)
+          end
+
+          it 'does not include production job' do
+            expect(build_names).not_to include('production')
+          end
+        end
       end
 
-      context 'when project has an Agent is present' do
+      context 'when project has an Agent' do
         before do
           create(:cluster_agent, project: project)
         end
 
         it_behaves_like 'pipeline with Kubernetes jobs'
+
+        context 'when certificate_based_clusters FF is disabled' do
+          before do
+            stub_feature_flags(certificate_based_clusters: false)
+          end
+
+          it 'includes production job' do
+            expect(build_names).to include('production')
+          end
+        end
       end
     end
 

spec/models/concerns/deployment_platform_spec.rb

@@ -12,16 +12,28 @@ RSpec.describe DeploymentPlatform do
       let(:group) { create(:group) }
       let(:project) { create(:project, group: group) }
 
+      shared_examples 'certificate_based_clusters is disabled' do
+        before do
+          stub_feature_flags(certificate_based_clusters: false)
+        end
+
+        it { is_expected.to be_nil }
+      end
+
       shared_examples 'matching environment scope' do
         it 'returns environment specific cluster' do
           is_expected.to eq(cluster.platform_kubernetes)
         end
+
+        it_behaves_like 'certificate_based_clusters is disabled'
       end
 
       shared_examples 'not matching environment scope' do
         it 'returns default cluster' do
           is_expected.to eq(default_cluster.platform_kubernetes)
         end
+
+        it_behaves_like 'certificate_based_clusters is disabled'
       end
 
       context 'multiple clusters use the same management project' do