Pass playbook filename to CallbackModule.playbook_on_start

Having just implemented a callback plugin to send notifications when ansible is being run, I found that not having the playbook filename available to playbook_on_start was potentially confusing, as you wouldn't really know what playbook was being run.

This pull request adds a play argument to playbook_on_start which contains the playbook filename.

My only "concern" here is that adding this will cause ansible to send a 2nd argument to the method, and those that already have a callback plugin implemented, will run into issues unless they update.