Add project level config for merge train

We add the option and expose the parameter for frontend work

db/migrate/20190416185130_add_merge_train_enabled_to_ci_cd_settings.rb

+# frozen_string_literal: true
+
+class AddMergeTrainEnabledToCiCdSettings < ActiveRecord::Migration[5.1]
+  include Gitlab::Database::MigrationHelpers
+
+  DOWNTIME = false
+
+  disable_ddl_transaction!
+
+  def up
+    add_column_with_default :project_ci_cd_settings, :merge_trains_enabled, :boolean, default: false, allow_null: false
+  end
+
+  def down
+    remove_column :project_ci_cd_settings, :merge_trains_enabled
+  end
+end

db/schema.rb

@@ -1612,6 +1612,7 @@ ActiveRecord::Schema.define(version: 20190426180107) do
     t.integer "project_id", null: false
     t.boolean "group_runners_enabled", default: true, null: false
     t.boolean "merge_pipelines_enabled"
+    t.boolean "merge_trains_enabled", default: false, null: false
     t.index ["project_id"], name: "index_project_ci_cd_settings_on_project_id", unique: true, using: :btree
   end
 

spec/models/merge_request_spec.rb

@@ -2123,7 +2123,7 @@ describe MergeRequest do
     end
 
     context 'when merges are not restricted to green builds' do
-      subject { build(:merge_request, target_project: build(:project, only_allow_merge_if_pipeline_succeeds: false)) }
+      subject { build(:merge_request, target_project: create(:project, only_allow_merge_if_pipeline_succeeds: false)) }
 
       context 'and a failed pipeline is associated' do
         before do