Merge pull request #3882 from bhcleek/doc-ansible-command

document ansible provisioner's command option.

website/source/docs/provisioners/ansible.html.md

@@ -47,6 +47,9 @@ Required Parameters:
 
 Optional Parameters:
 
+- `command` (string) - The command to invoke ansible.
+   Defaults to `ansible-playbook`.
+
 - `groups` (array of strings) - The groups into which the Ansible host
   should be placed. When unspecified, the host is not associated with any
   groups.