Extract CI entry config hash validation to validator

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

@@ -19,13 +19,7 @@ module Gitlab
           included do
             validations do
-              validate :hash_config_value
-
-              def hash_config_value
-                unless self.config.is_a?(Hash)
-                  errors.add(:config, 'should be a configuration entry hash')
-                end
-              end
+              validates :config, hash: true
             end
           end

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

@@ -12,6 +12,14 @@ module Gitlab
               end
             end
           end
+
+          class HashValidator < ActiveModel::EachValidator
+            def validate_each(record, attribute, value)
+              unless value.is_a?(Hash)
+                record.errors.add(attribute, 'should be a configuration entry hash')
+              end
+            end
+          end
         end
       end
     end