Add CI for 16x ubuntu servers (#6040)

tests/files/vagrant_ubuntu18-weave-medium.rb

@@ -0,0 +1,6 @@
+$num_instances = 16
+$vm_memory ||= 1600
+$os = "ubuntu1804"
+$network_plugin = "weave"
+$kube_master_instances = 1
+$etcd_instances = 1

tests/scripts/vagrant_clean.sh

@@ -3,6 +3,8 @@ set -euxo pipefail
 
 # Cleanup vagrant VMs to avoid name conflicts
 
+apt-get install -y libvirt-clients
+
 for i in $(virsh list --name)
 do
     virsh destroy "$i"