Improve the list of of playbooks

The box list_playbook lists all .yml files under the playbooks directory:

But the ldh_middleware playbook includes both the ldh_middleware.yml and all the files inside the ldh_middleware folder. I modified the box list_playbook command so it only lists the toplevel yml files.

The output comes from this:

$ shipwright list_playbooks
/home/nnieto/.config/ldh_developer/playbooks:
ldh_developer

/home/nnieto/.config/ldh_developer/playbooks/ldh_developer:
ldh_middleware
ldh_middleware.yml
README.md
shop.retry
shop.yml

/home/nnieto/.config/ldh_developer/playbooks/ldh_developer/ldh_middleware:
defaults
handlers
meta
README.md
tasks
templates
tests
vars

/home/nnieto/.config/ldh_developer/playbooks/ldh_developer/ldh_middleware/defaults:
main.yml

/home/nnieto/.config/ldh_developer/playbooks/ldh_developer/ldh_middleware/handlers:
main.yml

/home/nnieto/.config/ldh_developer/playbooks/ldh_developer/ldh_middleware/meta:
main.yml

/home/nnieto/.config/ldh_developer/playbooks/ldh_developer/ldh_middleware/tasks:
main.yml

/home/nnieto/.config/ldh_developer/playbooks/ldh_developer/ldh_middleware/templates:
ldh_middleware
nginx

/home/nnieto/.config/ldh_developer/playbooks/ldh_developer/ldh_middleware/templates/ldh_middleware:
config.ini.j2
secret.ini.j2

/home/nnieto/.config/ldh_developer/playbooks/ldh_developer/ldh_middleware/templates/nginx:
purist_middleware.j2

/home/nnieto/.config/ldh_developer/playbooks/ldh_developer/ldh_middleware/tests:
inventory
test.yml

/home/nnieto/.config/ldh_developer/playbooks/ldh_developer/ldh_middleware/vars:
main.yml

... to this:

$ shipwright list_playbooks
ldh_developer/shop.yml
ldh_developer/ldh_middleware.yml

I think this improves UX