Add gitlab ci configuration class that holds hash

As for now, we keep this class inside a oryginal config processor class.
We will move implementation to this class and delegate to it from
current config processor.

After original gitlab ci yaml processor not longer has relevant
impelemntation we will replace it with new configuration class.

lib/ci/gitlab_ci_yaml_processor.rb

@@ -12,18 +12,14 @@ module Ci
     attr_reader :before_script, :after_script, :image, :services, :path, :cache
 
     def initialize(config, path = nil)
-      @config = YAML.safe_load(config, [Symbol], [], true)
+      @config = Gitlab::Ci::Config.new(config).to_hash
       @path = path
 
-      unless @config.is_a? Hash
-        raise ValidationError, "YAML should be a hash"
-      end
-
-      @config = @config.deep_symbolize_keys
-
       initial_parsing
 
       validate!
+    rescue Gitlab::Ci::Config::ParserError => e
+      raise ValidationError, e.message
     end
 
     def builds_for_stage_and_ref(stage, ref, tag = false, trigger_request = nil)

lib/gitlab/ci/config.rb

+module Gitlab
+  module Ci
+    class Config
+      class ParserError < StandardError; end
+
+      def initialize(config)
+        @config = YAML.safe_load(config, [Symbol], [], true)
+
+        unless @config.is_a?(Hash)
+          raise ParserError, 'YAML should be a hash'
+        end
+
+        @config = @config.deep_symbolize_keys
+      end
+
+      def to_hash
+        @config
+      end
+    end
+  end
+end

spec/lib/gitlab/ci/config_spec.rb

+require 'spec_helper'
+
+describe Gitlab::Ci::Config do
+  let(:config) do
+    described_class.new(yml)
+  end
+
+  context 'when yml config is valid' do
+    let(:yml) do
+      <<-EOS
+        image: ruby:2.2
+
+        rspec:
+          script:
+            - gem install rspec
+            - rspec
+      EOS
+    end
+
+    describe '#to_hash' do
+      it 'returns hash created from string' do
+        hash = {
+          image: 'ruby:2.2',
+          rspec: {
+            script: ['gem install rspec',
+                     'rspec']
+          }
+        }
+
+        expect(config.to_hash).to eq hash
+      end
+    end
+  end
+end