Make Vagrant CI use unsafe I/O (#6058)

tests/files/vagrant_ubuntu18-flannel.rb

+# For CI we are not worries about data persistence across reboot
+$libvirt_volume_cache = "unsafe"
+
+# Checking for box update can trigger API rate limiting
+# https://www.vagrantup.com/docs/vagrant-cloud/request-limits.html
+$box_check_update = false
\ No newline at end of file