Rework vagrant inventory handling + support for multiples inventories

The current way to handle a custom inventory in vagrant is a bit
hackish, copy files around and can break Vagrantfile parsing in
cornercase scenarios (removing vagrant inventories, or the inventory
copied into vagrant inventory).

Instead, simply pass additional inventories to the ansible-playbook
command lines as raw arguments with `-i`.
This also makes supporting multiples inventories trivial, so we add a
new `$inventories` variable for that purpose.