Ability to delete build working directory after a failed build

While BAM-5192 is great for saving disk space, when builds start failing in large numbers, the last thing we want is for our agents to start filling up with old build results. It would be great if Bamboo could remove things more aggressively.

At a minimum, it would be great to delete all build results instead of just successful ones. I'm attaching a patch that does just that.