Adds feature flag for Group Clusters

As the frontend is still in progress and other related backend is still
in prgress we need a feature flag to turn off Group Clusters.

app/controllers/groups/clusters_controller.rb

@@ -3,6 +3,7 @@
 class Groups::ClustersController < Clusters::ClustersController
   include ControllerWithCrossProjectAccessCheck
 
+  prepend_before_action :check_group_clusters_feature_flag!
   prepend_before_action :group
   requires_cross_project_access
 
@@ -17,4 +18,8 @@ class Groups::ClustersController < Clusters::ClustersController
   def group
     @group ||= find_routable!(Group, params[:group_id] || params[:id])
   end
+
+  def check_group_clusters_feature_flag!
+    render_404 unless Feature.enabled?(:group_clusters)
+  end
 end

app/helpers/groups_helper.rb

@@ -140,7 +140,7 @@ module GroupsHelper
       can?(current_user, "read_group_#{resource}".to_sym, @group)
     end
 
-    if can?(current_user, :read_cluster, @group)
+    if can?(current_user, :read_cluster, @group) && Feature.enabled?(:group_clusters)
       links << :kubernetes
     end
 

spec/controllers/groups/clusters_controller_spec.rb

@@ -20,6 +20,23 @@ describe Groups::ClustersController do
       get :index, params.reverse_merge(group_id: group)
     end
 
+    context 'when feature flag is not enabled' do
+      before do
+        stub_feature_flags(group_clusters: false)
+      end
+
+      it 'renders 404' do
+        go
+
+        expect(response).to have_gitlab_http_status(404)
+      end
+    end
+
+    context 'when feature flag is enabled' do
+      before do
+        stub_feature_flags(group_clusters: true)
+      end
+
       describe 'functionality' do
         context 'when group has one or more clusters' do
           let(:group) { create(:group) }
@@ -62,6 +79,7 @@ describe Groups::ClustersController do
           end
         end
       end
+    end
 
     describe 'security' do
       let(:cluster) { create(:cluster, :provided_by_gcp, cluster_type: :group_type, groups: [group]) }