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).