post-processor/vagrant: pass PackerConfig to sub post-processors

Populates `PackerConfig` values into extra config provided to sub-post-processors, which is necessary for allowing the `output` value to be a configuration template.  

This should be the last bit to resolve #413, based on the comments I made to bda4754e686aee3746fbde35ac4737570489bf50. /cc @mitchellh