Support addons with continuous delivery

[efekarakus]

Existing system

Here is our existing CodePipeline layout.

Within the build stage of the pipeline, we run app package --output-dir infrastructure to generate the CloudFormation template from the manifest file and addons template (see #648)

Once the build stage completes, the infrastructure/ directory is added as an artifact to the PipelineBuiltArtifactBucket.

Required changes

1. We need to set the appropriate value for AddonsTemplateURL in the Test stage so that the template url for the nested stack can be constructed in the application stack. We're proposing to leverage Fn::GetArtifactAtt with ParameterOverrides in each environment stage to retrieve the URL of the output artifact from the Build stage. So we would need to make the following change to pipeline_cfn.yml:

ParameterOverrides: |
  {
    "AddonsTemplateURL" : { "Fn::Join" : [ "/", [ "Fn::GetArtifactAtt" : ["BuildOutput", "URL"], "infrastructure", "{{$app}}.addons.stack.yml " ] ] }
  }

2. However, we can't set the ParameterOverrides if the application does not have any addons!
   Therefore, we need to modify stack/pipeline.go to pass information to denote that a particular application is expected to have addons.

Parent story

#644