Update "ecs-project" directory to support addons

[efekarakus]

Ask

We need to update the ecs-project/ directory in a workspace to instead create manifest files under an {appName}/ directory.

./
├── Dockerfile
├── LICENSE
├── app.js
└── ecs-project/
    └── {appName}/
        └── manifest.yml

Note that the manifest file is now renamed from {appName}-app.yml to only manifest.yml. This way we're using the same terminology as in our documentation when referring to the file.

Current system

./
├── Dockerfile
├── LICENSE
├── app.js
└── ecs-project/
    └── {appName}-app.yml

Parent story

This change will allow us to add an addons/ directory under the {appName}/ directory:
#644