Add vagrant cachier plugin

https://github.com/fgrehm/vagrant-cachier

Should speed up builds if installed with vagrant plugin install
vagrant-cachier. Shouldnt affect things if this plugin is not installed.
Don't enforce vmware memory / cpu

If this is set in the main Vagrantfile, it cannot be overridden in
~/.vagrant.d/Vagrantfile
Merge pull request #655 from alphagov/vagrantfile-changes

Dont force Vmware options / Add cachier plugin config