Fix passing the purge time set in snippet

I had an issue with backups not being kept until their purge time.
After setting the databackup to run with CronManager (and using the default purge time of 21 days) a backup was being created every 24 hours, but only the most recent one was being kept - so only 1 backup copy. 

I investigated the snippet to see what the purge time was and found the purge property in the snippet was not being passed properly. I've fixed this in this commit