roles: bifrost-create-vm-nodes: Set NIC model to 'virtio'

The role already uses the virtio driver to improve the disk peformance.
We can also use the virtio driver for the NIC device in order to improve
the network performance of the test VM. However, users may want to
change that so we are adding a new test_vm_nic Ansible variable for
that.

Change-Id: I9e5c9a33f1f6903d39d0fa2721288d084071e733

playbooks/roles/bifrost-create-vm-nodes/defaults/main.yml

@@ -6,6 +6,7 @@ test_vm_num_nodes: 1
 test_vm_domain_type: "qemu"
 test_vm_arch: "x86_64"
 test_vm_cpu: "host-model"
+test_vm_nic: "virtio"
 test_vm_groups: {}
 test_vm_default_groups: "{{ lookup('env', 'DEFAULT_HOST_GROUPS').split() | default(['baremetal'], true) }}"
 test_vm_disk_gib: "{{ lookup('env', 'VM_DISK') | default(10, true) }}"

playbooks/roles/bifrost-create-vm-nodes/templates/testvm.xml.j2

@@ -33,6 +33,7 @@
     </controller>
     <interface type='network'>
       <source network='{{ test_vm_network }}'/>
+      <model type='{{ test_vm_nic }}'/>
     </interface>
     <input type='mouse' bus='ps2'/>
     <graphics type='vnc' port='-1' autoport='yes'/>

releasenotes/notes/vm-nic-type-support-0ac47ccd3d56132c.yaml

@@ -0,0 +1,7 @@
+---
+features:
+  - |
+    Add support for tweaking the vNIC model for the VMs created
+    by bifrost-create-vm-nodes. The default vNIC model is 'virtio'
+    which should provide the best possible performance. The model can be
+    changed using the `test_vm_nic` Ansible variable.