Enable frozen string across project (#1319)

Turn on frozen string support in all files by using a comment to avoid
enabling across dependencies where it may not work.

Fixes: #1177

tests/two_disks/Vagrantfile

@@ -1,5 +1,7 @@
 # -*- mode: ruby -*-
 # vi: set ft=ruby :
+#
+# frozen_string_literal: true
 
 Vagrant.configure("2") do |config|
   config.vm.box = "infernix/tinycore-two-disks"