[DEPLOY-CI] Insert jinja2 rendering for deploy CI

* Since the number of custom projects and thus of GitLab groups and
repositories to which the CI/CD steps need to be deployed, jinja2
rendering is introduced in the same way as it was done for the manifest
integration steps.
* Define the INTEGRATION_CI project variable for this repository in
order to define a list of group, manifest repository and branch name to
which the gitlab-ci shall be integrated.
* Use the mechanism of dynamic child pipeline creation.
* Update documentation according to the modifications.