Problem: when fixity is installed, a fixity cron job is enabled by default, cron frequency is hardcoded

[hakamine]

When fixity is installed (when variable archivematica_src_install_fixity is set to yes), it also enables a cron job to run fixity periodically, and the cron job frequency is hardcoded. It would be better to make both configurable in the playbook (cron job enable/disable, cron job frequency)