[PIPELINE] Insert cancel pipeline and yamllint job

* The cancel-previous-pipeline job aims at reducing dead time. Indeed,
everytime we push a new commit to a branch, a new pipeline is created.
Often the pipeline for the previous commit has not finished yet, and
GitLab does not automatically cancel it. It consumes valuable build
time, even if we don't need the results anymore. This adds a MR pipeline
job to search for previous pipelines on the same branch and cancel
them explicitly.
* Add yamllint job allowing to check for correct yaml syntax in the
dynamic child pipeline created in the layers and manifest integration
jobs.
* Add yamllint job also on .gitlab-ci pipeline.