Enfore the used Vagrantfile API/syntax version when not initialised

If there is no base Vagrantfile (~/.vagrant.d/Vagrantfile) it's not
possible to start the box because VAGRANTFILE_API_VERSION is not
initialised by default.

Initializing VAGRANTFILE_API_VERSION when not set will resolve the
following issue: "uninitialized constant VAGRANTFILE_API_VERSION".

Change-Id: Ic3dc7b7e78b2634ad902a713cdac02ace71dce1d
Merge "Enfore the used Vagrantfile API/syntax version when not initialised"