Use version-less openbsd names

OpenBSD system is developed at head on a regular cadence without any semantic versioning. The next version 7.0 will arrive this fall. We might as well not enshrine the number.

openbsd/Vagrantfile

 # vim: filetype=ruby
 
 Vagrant.configure("2") do |config|
-  config.vm.box = "generic/openbsd6"
+  config.vm.box = "generic/openbsd"
   config.vm.guest = :openbsd
   config.ssh.shell = "sh"
   config.vm.synced_folder ".", "/vagrant", id: "vagrant-root", disabled: true