Use deploy_steps_tasks instead of external_deploy_tasks so we execute
the playbook on nodes that match "overcloud" group, which is the case of
the containerized undercloud.
Also add deploy_steps_tasks to tools/yaml-validate.py part of
OPTIONAL_DOCKER_SECTIONS.
Change-Id: Iaa6a05bf864cdb54a000ef74e6c5ab8e627ab0cf