Add a 'box' Vagrantfile template

[arm4b]

This Vagrantfile is shipped with the box (bundled into an image) and used after unpacking the box during vagrant commands.

User's Vagrantfile will be merged with this custom config, giving priority to user's settings.

For more info about the original Vagrantfile load-order-merge mechanism see:

• https://groups.google.com/forum/#!topic/packer-tool/Iu1jSRgbWOk
• https://www.vagrantup.com/docs/cli/package.html
• https://www.vagrantup.com/docs/vagrantfile/#load-order

[arm4b]

Created another issue we'll need to address https://github.com/StackStorm/packer-st2/issues/17 during this work, - warn user about insecure login credentials used for both Vagrant/OVA.