allow overriding vagrant box in test Vagrantfile via ENV variable

Author: pbitty

test/Vagrantfile

@@ -6,8 +6,13 @@ if Gem::Version.new(::Vagrant::VERSION) < Gem::Version.new('1.5')
 end
 
 Vagrant.configure('2') do |config|
-  config.vm.box = 'precise64'
-  config.vm.box_url = 'http://cloud-images.ubuntu.com/precise/current/precise-server-cloudimg-vagrant-amd64-disk1.box'
+
+  if ENV.key? 'VAGRANT_BOX'
+    config.vm.box = ENV['VAGRANT_BOX']
+  else
+    config.vm.box = 'precise64'
+    config.vm.box_url = 'http://cloud-images.ubuntu.com/precise/current/precise-server-cloudimg-vagrant-amd64-disk1.box'
+  end
 
   config.hostmanager.enabled = true
   config.hostmanager.manage_host = true