Create an empty 'authorized_keys' file

Solves issue https://github.com/rcbops/ansible-lxc-rpc/issues/478

`touch`-es an empty file with the necessary name before an output redirect to it. This works around the Ansible limitation of not creating files that don't explicitly exist.
Ah. If we're going to do that, I'd go one further. Use the authorized_key module.