Add domain field into Clusters::Cluster

This is the 1st step for moving Auto DevOps domain into cluster
settings, whether is project or group.

Related to https://gitlab.com/gitlab-org/gitlab-ce/issues/52363

app/models/clusters/cluster.rb

@@ -49,8 +49,9 @@ module Clusters
 
     validates :name, cluster_name: true
     validates :cluster_type, presence: true
-    validate :restrict_modification, on: :update
+    validates :domain, allow_nil: true, hostname: { allow_numeric_hostname: true, require_valid_tld: true }
 
+    validate :restrict_modification, on: :update
     validate :no_groups, unless: :group_type?
     validate :no_projects, unless: :project_type?
 

db/migrate/20190114172110_add_domain_to_cluster.rb

+# frozen_string_literal: true
+
+class AddDomainToCluster < ActiveRecord::Migration[5.0]
+  DOWNTIME = false
+
+  def change
+    add_column :clusters, :domain, :string
+  end
+end

db/schema.rb

@@ -647,6 +647,7 @@ ActiveRecord::Schema.define(version: 20190115054216) do
     t.string "name", null: false
     t.string "environment_scope", default: "*", null: false
     t.integer "cluster_type", limit: 2, default: 3, null: false
+    t.string "domain"
     t.index ["enabled"], name: "index_clusters_on_enabled", using: :btree
     t.index ["user_id"], name: "index_clusters_on_user_id", using: :btree
   end

spec/factories/clusters/clusters.rb

@@ -59,5 +59,9 @@ FactoryBot.define do
     trait :with_installed_helm do
       application_helm factory: %i(clusters_applications_helm installed)
     end
+
+    trait :with_domain do
+      domain 'example.com'
+    end
   end
 end

spec/models/clusters/cluster_spec.rb

@@ -113,7 +113,7 @@ describe Clusters::Cluster do
     end
   end
 
-  describe 'validation' do
+  describe 'validations' do
     subject { cluster.valid? }
 
     context 'when validates name' do
@@ -252,6 +252,31 @@ describe Clusters::Cluster do
         end
       end
     end
+
+    describe 'domain validation' do
+      let(:cluster) { build(:cluster) }
+
+      subject { cluster }
+
+      context 'when cluster has domain' do
+        let(:cluster) { build(:cluster, :with_domain) }
+
+        it { is_expected.to be_valid }
+      end
+
+      context 'when cluster has an invalid domain' do
+        let(:cluster) { build(:cluster, domain: 'not-valid-domain') }
+
+        it 'should add an error on domain' do
+          expect(subject).not_to be_valid
+          expect(subject.errors[:domain].first).to eq('is not a fully qualified domain name')
+        end
+      end
+
+      context 'when cluster does not have a domain' do
+        it { is_expected.to be_valid }
+      end
+    end
   end
 
   describe '.ancestor_clusters_for_clusterable' do