Support for Parallels Desktop

Parallels Desktop offers an official and open-source Vagrant Provider plugin at http://parallels.github.io/vagrant-parallels/ and https://github.com/Parallels/vagrant-parallels.
The only real differences to our Virtualbox configuration that I have spotted after a quick glance are the box name and the way how machine settings are changed. Both of this can be overridden with provider-specific declarations in the Vagrantfile, as far as I can see.

I'll use this as a tracking issue for my experiments with adding support to VVV.