Add a property to opt-out for 'debug.log', making it optional

Currently, the debug logs in `/var/vcap/sys/log/cassandra/debug.log` are activated by default and can't be opted-out. Plus, `INFO`-level logs are copied twice in both `debug.log` and `system.log`. As a performance point of view, this could be optimized.

A new job property for turning these debug logs off would be great.