Add waiting mechanism for assets image from CE/EE projects to CNG pipeline
Towards #942 (closed)
With CNG releases happening from tag pushes to CNG project (rather than trigger from CE/EE projects), that immediately follow tag pushes to CE/EE projects, we need a waiting mechanism that waits for the asset image from CE/EE being available before starting building images in CNG. This need to happen in the stage before gitlab-rails docker image is built.