Code review comment for ~jocha/cloud-init:jocha-reboot-bugfix-2

Last pass I promise. What do you think about this. http://paste.ubuntu.com/p/MsVK5PZZ98/

let's avoid leaking REPORTED_READY_MARKER_FILE outside of poll_imds because poll_imds is the only thing that cares about whether or not this marker exists. It simplifies the poll_imds arg list and avoid unnecessary complexity outside of this method.