Set the default interface for bridged networks to avoid interactive 'vagrant up'.

Thx to https://stackoverflow.com/questions/33250304/how-to-automatically-select-bridged-network-interfaces-in-vagrant

tests/vagrant/funcs.rb

@@ -13,3 +13,5 @@ def use_preloaded_box(obj, name, preloaded_dir=".")
   end
 end
 
+# Grab the name of the default interface
+$default_network_interface = `ip route | awk '/^default/ {printf "%s", $5; exit 0}'`

tests/vagrant/vanilla/Vagrantfile

@@ -3,7 +3,7 @@ load '../funcs.rb'
 Vagrant.configure("2") do |config|
 
   config.vm.synced_folder "../../..", "/mailinabox", id: "mailinabox", automount: false
-  config.vm.network "public_network"
+  config.vm.network "public_network", bridge: "#$default_network_interface"
   use_preloaded_box config, "ubuntu/bionic64", ".."
 
   # vanilla install