https://gitlab.com/gitlab-org/gitlab-ce/issues/65019

This commit adds CI job that validates all `*.yml`
with `yamllint`.

This commit fixes all offenses present in repository.

This allows these templates to be used outside of Auto DevOps,
or easily excluded from Auto DevOps.

The security jobs in Auto DevOps differ slightly from the ones
that are already available as templates, they will be moved
next.