ansible setup.yml failed with error ''ansible_distribution' is undefined '

Hi
I want to install Openwhisk with _docker_ using _ansible_. But when I run `ansible-playbook -i environments/local setup.yml` I get this error:

> The conditional check 'nginx.ssl.cert == "openwhisk-server-cert.pem"'
failed. The error was: error while evaluating conditional (nginx.ssl.cert ==
"openwhisk-server-cert.pem"): 'ansible_distribution' is undefined The error
appears to have been in '/home/parichehr/openwhisk/ansible/setup.yml': line 60,
column 5, but may be elsewhere in the file depending on the exact syntax
problem. The offending line appears to be: # Generate nginx certificates -
name: gen untrusted server certificate for host ^ here