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

spec/unit/action/create_domain_volume_spec.rb

@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 require 'spec_helper'
 require 'support/sharedcontext'
 require 'support/libvirt_context'