Remove "named playbook sets" from roadmap.

While it seemed convenient to have named playbook sets directly in a
stack config, it seems more manageable to have a separate yml file to
define playbook sets.  E.g.:

    bang-stacks/
               |_ prod-stack.yml
               |_ prod-playbooks-deploy.yml
               |_ prod-playbooks-update_lbs.yml
               |_ dev-overrides.yml

Where prod-playbooks-deploy.yml would contain:

    playbooks:
    - deploy_dbs.yml
    - sanity_check_dbs.yml
    - deploy_app_servers.yml
    - sanity_check_stack.yml

And prod-playbooks-update_lbs.yml would contain:

    playbooks:
    - enable_maintenance_window.yml
    - update_lbs.yml
    - disable_maintenance_window.yml

Use cases:

    - Deploying a brand new prod stack:

        bang prod-stack.yml prod-playbooks-deploy.yml

    - Deploying a brand new dev stack:

        bang prod-stack.yml prod-playbooks-deploy.yml dev-overrides.yml

    - Updating load balancers in prod:

        bang prod-stack.yml prod-playbooks-update_lbs.yml