Code review comment for ~oddbloke/cloud-init/+git/cloud-init:lp1843276

OK, I've updated the log message to indicate we're writing to a temporary file, and added a log message for the rename (because I think failures could happen in either of those steps, and this allows us to distinguish between those). (We will also have the different logger name to distinguish between the two log messages, which is why I didn't push even more context into the log message itself.)

Thanks for the review, Scott!