Code review comment for ~ack/maas:test-results-limits

This branch doesn't account for Physical block devices. As I describe below this means systems with more then 10 disks will be broken.

While we've agreed to keep storage limits for now I think we should give the user an option to store unlimited results. I would say if max_node_commissioning_results, max_node_testing_results, or max_node_installation_results are set to 0 keep unlimited results.