Setting auto mode by default for worker_processes parameter

Hi,

It would be nice to set the auto mode if nothing is set as described in the official documentation (http://nginx.org/en/docs/ngx_core_module.html#worker_processes). If you don't want to use auto because it requires a certain version, you may use this instead:
```
worker_processes {{ nginx_workers_processes | default(ansible_processor_cores) }};
```
What do you think about it ?