added README and args to playbook

ansible/README.md

@@ -0,0 +1,5 @@
+Ping host (host label from inventory)
+```shell
+ansible-playbook --extra-vars "hosts=<hosts>" playbooks/ping_workers.yml
+```
+

ansible/playbooks/ping_workers.yml

@@ -1,5 +1,5 @@
 - name: Ping workers
-  hosts: workers
+  hosts: "{{ hosts }}"
   gather_facts: False
   roles:
     - { role: ../roles/ping }