Add VirtualBox minimum version warning to Vagrantfile

The documentation mentions that "VirtualBox provider, version 4.3.10 or greater is required.".

When testing CoreOs with an older version of VirtualBox I had difficulties getting etcd communicate in the cluster. After upgrading to VirtualBox 4.3.10 the issues went away.

This pull-request adds a warning when the VirtualBox provider minimum version is not met. Sadly it's not possible to get the provider used from inside the Vagrantfile (https://github.com/mitchellh/vagrant/issues/1867) so the warning will show even if the vmware provider is used (and VirtualBox installed).


This block of code is executed when the vmware provider is used, so no it shouldn't stop block users from using vmware when they happen to have an older version of VirtualBox.