Add ci config class that represents a key value

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

+module Gitlab
+  module Ci
+    class Config
+      module Node
+        ##
+        # Entry that represents a key.
+        #
+        class Key < Entry
+          include Validatable
+
+          validations do
+            validates :config, key: true
+          end
+        end
+      end
+    end
+  end
+end

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

@@ -13,6 +13,16 @@ module Gitlab
             end
           end
+          class KeyValidator < ActiveModel::EachValidator
+            include LegacyValidationHelpers
+
+            def validate_each(record, attribute, value)
+              unless validate_string(value)
+                record.errors.add(attribute, 'should be a string or symbol')
+              end
+            end
+          end
+
           class TypeValidator < ActiveModel::EachValidator
             def validate_each(record, attribute, value)
               type = options[:with]

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

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