Improve the interface for the CbngWebService.
This merge request follows on from !51 (merged) and !50 (merged) to:
- Bring together the removal of JARs into the same classmethod as the download functionality
- This allows a public API to download a bunch of artifacts without needing to resolve them first (if you know what artifacts you need). This will be used in acc-py-deploy for example.
- Clarify that the CbngWebService tests are integration tests (they hit a real (local) server) by moving them to an appropriat e directory
- Update the conftest to only allow integration test like fixtures if you are in the integration directory
- Expose a public Artifact class in cbng_web_resolver, rather than passing around dictionaries
- Rename (deprecate) the old "list" command and call it "registered_packages"
cc @pbudzyns