Add "--daily" flag to LogSaver

The daily flag marks a backup as one of our daily backups by adding
"DAILY_" to the beginning of the tarball's name. This allows us to use a
script to clean up backups while preserving anything marked with
"DAILY_".

For example, the following find command will delete backup tarballs
are older than 25 hours unless they are marked with the daily flag:

find ~/CMS/hcal_pm_tester_logs/ -name "power_mezzanine_tester_logs_*"
-type f -mmin +1500 -delete