Add create_parallel option to create VMs parallel.

Since v1.4.0, Vagrant supports creating VMs parallel.
If `create_parallel` option is `true` and the version of vagrant is v1.4.0 or later, kitchen-vagrant creates VMs parallel.

http://www.vagrantup.com/blog/vagrant-1-4.html
> Running multiple "vagrant up" commands in parallel with VirtualBox is now safe. This is a boon for CI or automated environments.