Support YAML alias/anchor usage in .gitlab-ci.yml

This allows to reuse one job as a template for another one:

```
job1: &JOBTMPL
  script: execute-script-for-job

job2: *JOBTMPL

```

This also helps to solve some of the issues in #342
Signed-off-by: Pascal Bach <pascal.bach@siemens.com>
Signed-off-by: Fabio Huser <fabio.huser@siemens.com>

lib/ci/gitlab_ci_yaml_processor.rb

@@ -10,7 +10,7 @@ module Ci
     attr_reader :before_script, :image, :services, :variables, :path, :cache
 
     def initialize(config, path = nil)
-      @config = YAML.safe_load(config, [Symbol])
+      @config = YAML.safe_load(config, [Symbol], [], true)
       @path = path
 
       unless @config.is_a? Hash

spec/lib/ci/gitlab_ci_yaml_processor_spec.rb

@@ -427,6 +427,45 @@ module Ci
       end
     end
 
+    describe "YAML Alias/Anchor" do
+      it "is correctly supported for jobs" do
+        config = <<EOT
+job1: &JOBTMPL
+  script: execute-script-for-job
+
+job2: *JOBTMPL
+EOT
+
+        config_processor = GitlabCiYamlProcessor.new(config)
+
+        expect(config_processor.builds_for_stage_and_ref("test", "master").size).to eq(2)
+        expect(config_processor.builds_for_stage_and_ref("test", "master").first).to eq({
+          except: nil,
+          stage: "test",
+          stage_idx: 1,
+          name: :job1,
+          only: nil,
+          commands: "\nexecute-script-for-job",
+          tag_list: [],
+          options: {},
+          when: "on_success",
+          allow_failure: false
+        })
+        expect(config_processor.builds_for_stage_and_ref("test", "master").second).to eq({
+          except: nil,
+          stage: "test",
+          stage_idx: 1,
+          name: :job2,
+          only: nil,
+          commands: "\nexecute-script-for-job",
+          tag_list: [],
+          options: {},
+          when: "on_success",
+          allow_failure: false
+        })
+      end
+    end
+
     describe "Error handling" do
       it "fails to parse YAML" do
         expect{GitlabCiYamlProcessor.new("invalid: yaml: test")}.to raise_error(Psych::SyntaxError)