Parallels Desktop Linux boxes

Since Packer 0.6.0 it supports building boxes for [Parallels Desktop](http://www.parallels.com/eu/products/desktop/). I have adapted all Linux boxes to also be able to build for Parallels Desktop. I can help keeping it up to date.

I have published some builds on [vagrantcloud](https://vagrantcloud.com/rickard-von-essen).