Configurable .vagrant directory location

[kri5]

What would you like to be added:

It would be nice to be able to configure the .vagrant dir location in the Vagrantfile.

Why is this needed:

It is necessary in order to allow support for use-cases such as wrapper repo, where you would "include" kubespray in your existing ansible repo, but also with a Vagrantfile that would wrap the kubespray one.

In such use cases, we end up with a .vagrant inventory location located inside kubespray folder, where we would want the .vagrant directory to be located in the wrapper repo folder.