Vagrant virtualbox worker env override fixes

* Change ENV var check to only validate the existence of the
  var in the local env
* Add VAGRANT_WORKER_SCALE env variable override to control
  GUI scaling for virtualbox

Signed-off-by: James W. Brinkerhoff <jwb@paravolve.net>

deploy/vagrant/Vagrantfile

@@ -1,9 +1,15 @@
 ENV['VAGRANT_NO_PARALLEL'] = 'yes'
 
-# Returns true if `GUI` environment variable is set to a non-empty value.
+# Returns true if `GUI` environment variable exists, value does not matter.
 # Defaults to false
 def worker_gui_enabled?
-  ENV.fetch('VAGRANT_WORKER_GUI', '').empty?
+  ENV.include?('VAGRANT_WORKER_GUI')
+end
+
+# Returns true if `SCALE` environment variable exists, value does not matter.
+# Defaults to false
+def worker_display_scale_enabled?
+  ENV.include?('VAGRANT_WORKER_SCALE')
 end
 
 Vagrant.configure('2') do |config|
@@ -57,6 +63,10 @@ Vagrant.configure('2') do |config|
       vb.memory = 4*1024
       vb.cpus = 1
       vb.gui = worker_gui_enabled?
+      vb.customize [
+        'setextradata', :id,
+        'GUI/ScaleFactor', '3.0'
+      ] if worker_display_scale_enabled?
       vb.customize [
         'modifyvm', :id,
         '--nic1', 'none',

test/_vagrant/vagrant_test.go

@@ -95,7 +95,6 @@ func TestVagrantSetupGuide(t *testing.T) {
 
 	t.Logf("WorkflowID: %s", workflowID)
 
-	os.Setenv("VAGRANT_WORKER_GUI", "false")
 	worker, err := vagrant.Up(ctx,
 		vagrant.WithLogger(t.Logf),
 		vagrant.WithMachineName("worker"),