Fix the Vagrantfile for Github runners

Vagrantfile

@@ -16,13 +16,9 @@ SSH_FORWARD  = settings['SSH_FORWARD']  || false
 
 # Default to shell environment variable: PLAYBOOK (priority #1)
 PLAYBOOK=ENV["PLAYBOOK"]
-if !PLAYBOOK
+if !PLAYBOOK || PLAYBOOK.empty?
   # PLAYBOOK setting in .vagrant.yml (priority #2)
-  PLAYBOOK = settings['PLAYBOOK'] || false
-  if !PLAYBOOK || PLAYBOOK.empty?
-    puts "[VAGRANTFILE ERROR]: Set PLAYBOOK setting in .vagrant.yml"
-    abort
-  end
+  PLAYBOOK = settings['PLAYBOOK'] || 'default'
 end
 
 Vagrant.configure("2") do |config|