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

https://bugs.launchpad.net/juju-core/+bug/1345553
thumper would hate this: hvstMeth :-)
I don't mind things like "cmd" etc, but in this case I would also prefer "harvestMethod"
I had connotations of cooking up some illegal drugs when I read hvstMeth.

If we agree to change the name to "mode", it would be "harvestMode"