Update deprecated 'play_hosts' with 'ansible_play_batch'

Ansible built-in variable 'play_hosts' has been deprecated since
release 2.2, and replaced by a new 'ansible_play_batch' variable
according to the official document below. We should reflect the
update to playbook site.yml.
link: http://docs.ansible.com/ansible/playbooks_variables.html

Change-Id: I937fe93ea893a3469019a0759ec57d2e91e8adab
Closes-Bug: #1694650

ansible/site.yml

@@ -29,7 +29,7 @@
       delegate_to: "{{ item }}"
       with_items: "{{ groups['all'] }}"
       when:
-        - (play_hosts | length) != (groups['all'] | length)
+        - (ansible_play_batch | length) != (groups['all'] | length)
 
 - name: Detect openstack_release variable
   hosts: all