CI: deploy_gitlab_ci: Rework the deployment of gitlab-ci into the child projects

The deploy_gitlab_ci now creates the integration commit and branch in
each passed subproject and create an integration commit in the manifest
containing all these new revisions.
A build is then triggered on this commit to test the functionality.

Split the update_submodule functions to reuse them in different ways.
Remove some previously used files.

BCS 746-000740