Add scaffold of CI config for the job stage entry

lib/gitlab/ci/config/node/stage.rb

+module Gitlab
+  module Ci
+    class Config
+      module Node
+        ##
+        # Entry that represents a stage for a job.
+        #
+        class Stage < Entry
+          include Validatable
+
+          validations do
+            validates :config, key: true
+          end
+
+          def self.default
+            :test
+          end
+        end
+      end
+    end
+  end
+end

spec/lib/gitlab/ci/config/node/stage_spec.rb

+require 'spec_helper'
+
+describe Gitlab::Ci::Config::Node::Stage do
+  let(:entry) { described_class.new(config) }
+
+  describe 'validations' do
+    context 'when entry config value is correct' do
+      let(:config) { :stage1 }
+
+      describe '#value' do
+        it 'returns a stage key' do
+          expect(entry.value).to eq config
+        end
+      end
+
+      describe '#valid?' do
+        it 'is valid' do
+          expect(entry).to be_valid
+        end
+      end
+
+      context 'when entry config is incorrect' do
+        let(:config) { { test: true } }
+
+        describe '#errors' do
+          it 'reports errors' do
+            expect(entry.errors)
+              .to include 'stage config should be a string or symbol'
+          end
+        end
+
+        describe '#valid?' do
+          it 'is not valid' do
+            expect(entry).not_to be_valid
+          end
+        end
+      end
+    end
+  end
+
+  describe '.default' do
+    it 'returns default stage' do
+      expect(described_class.default).to eq :test
+    end
+  end
+end