Specify mandatory name parameter for virt_net

This was documeted as required in ansible-2.9, but apparently only
enforced from ansible-2.10.

Change-Id: I8c71545e5e64c07cba60ac53e035d79c7c8459b0

playbooks/roles/bifrost-create-vm-nodes/tasks/prepare_libvirt.yml

@@ -92,6 +92,7 @@
 
 - name: find facts on libvirt networks
   virt_net:
+    name: "{{ test_vm_network }}"
     command: facts
     uri: "{{ test_vm_libvirt_uri }}"