Remember cron comment

Two small fixes to how existing & new cron entries are handled with regards to comments. Code was sometimes forgetting to pass along a comment, or not adding a blank one. Later code would try to read it and throw a KeyError.

Rather than make that code use .get('comment') and potentially hide future problems like this, I opted to explicitly add in a comment key to the cron dict, even if blank. Seems like a sensible contract with the lower level cron code in salt.