Per the documentation for builders, the "configure" or "prepare" method shouldn't have any side effects, such as this. Instead, we should create another step (maybe the first step that runs) that deletes the output directory if it already exists. 

This is important because for example, if you run `packer validate` and _somehow_ (I realize it is impossible now) the "packer_force" flag was set, it would delete the directory, as part of validation. Kind of surprising.

Same thing for VMware. Can you do this in another PR?