Support for more comprehensive harvesting mode vs binary safe-mode.

https://bugs.launchpad.net/juju-core/+bug/1345553
It's already reflected in the config processing, but I was trying to follow precedent with keeping safe-mode turned off here. I wasn't sure if the provisioner needs to do something special when it first starts off. Based on your comment, I've flipped this to HarvestDestroyed.