tests: support default value in vagrant boxes.

tests/Vagrantfile

@@ -6,6 +6,7 @@
 require 'yaml'
 
 machines = YAML.load_file(File.join(File.dirname(__FILE__), 'boxes.yml'))
+default = machines['defaults']
 
 Vagrant.require_version '>= 2.0.0'
 
@@ -33,9 +34,9 @@ Vagrant.configure("2") do |config|
         libvirt.driver            = 'kvm'
         libvirt.default_prefix    = 'aa-'
         libvirt.connect_via_ssh   = false
-        libvirt.storage_pool_name = 'default'
-        libvirt.memory            = instance['ram']
-        libvirt.cpus              = instance['cpu']
+        libvirt.storage_pool_name = 'ssd'
+        libvirt.memory            = instance.fetch('ram', default['ram'])
+        libvirt.cpus              = instance.fetch('cpu', default['cpu'])
         libvirt.cpu_mode          = 'host-passthrough'
         libvirt.machine_type      = 'q35'
         libvirt.video_type        = 'virtio'
@@ -51,7 +52,7 @@ Vagrant.configure("2") do |config|
         (1..2).each do
           libvirt.redirdev :type => "spicevmc"
         end
-        if instance['uefi']
+        if instance.fetch('uefi', default['uefi'])
           libvirt.loader = '/usr/share/edk2-ovmf/x64/OVMF_CODE.fd'
         end
       end