Enable plugin control via metadata in the cron blocks

[mrenvoize]

We should formalise a specification for metadata that can be added to cron "blocks" so that ansible/system maintained crontab files and plugin maintained cron config can interact more cleanly.

I'm thinking things like marking cronjobs as 'system/readonly', 'hidden', 'user ownership' etc

[mrenvoize]

Also need to make the API/Cron read/write a bit more bomb proof.. right now the example ansible setup makes my install explode and managing the same cron file via the plugin and an ansible layer needs to work nicely.

[mrenvoize]

I think this is the most important next step for the plugin. We need to have a more reliable way of parsing and working with the crontab files.

[lizrea]

Yes, love this