Fix Vagrant Post-processor builder specific configuration [GH-413]

Fixes issue where defining builder specific configuration for the Vagrant post-processor resulted in errors because of the output path was missing. Pull request also adds additional logging messages for the builder specific post-processors to help understand how they are being configured.

@mitchellh With this PR, the behavior originally reported in #414 still exists. However, a `vagrantfile_template` option is not acting as a default value to be overriden by builder specific settings, but instead is an override of any builder defined values. I wanted to get these changes out now, so decided not to tackle that item at the moment.