Making log file creation idempotent

Ensuring that making sure log file exists is truly idempotent, i.e. does not result in `changed` state after first run.

Previously as we were using the `touch` state of the [`file`](http://docs.ansible.com/file_module.html) module we were getting a `changed` state every time the playbook was run.

Tests passed because the log file was only touched if it was not `'""'` (as it is during tests).