REST API: Endpoint for downloading zipped project

[stefanbschneider]

For integration with the SDK portal and the packager, we'll need a REST API endpoint to download the files of a specific project (as a zip file). This zip file can then be sent to an endpoint of the packager for proper packaging. This is necessary because project and packager are running in separate containers.

I already started with a dummy implementation here: https://github.com/sonata-nfv/tng-sdk-project/blob/master/src/tngsdk/rest.py#L347

• [ ] Extend and debug the existing code to allow downloading zipped project files
• [ ] Write corresponding tests using Tavern (if possible; else plain pytest). See test here

[stefanbschneider]

This endpoint isn't needed anymore, since we'll run project and packager in the same container, without using the packager's REST API (instead just trigger it via the project's new endpoint: https://github.com/sonata-nfv/tng-sdk-project/pull/98)