Give `supplemental_node` default value

`supplemental_node` will not always be defined for a given OOOQ
invocation. Nested conditionals within blocks do not behave properly
so we have to allow for graceful failures. Adding a default, empty,
string here will prevent Ansible from hard failing on an undefined
variable.

Change-Id: I8bcdc5793bf2d25e8dd11f4684c642b74b2b2798

roles/libvirt/teardown/nodes/tasks/main.yml

@@ -61,19 +61,19 @@
       block:
       - name: Check undercloud vm
         command: >
-          virsh domid "{{ supplemental_node }}"
+          virsh domid "{{ supplemental_node.name|default('') }}"
         ignore_errors: true
         register: supplemental_check
 
       - name: Destroy supplemental vm
         command: >
-          virsh destroy "{{ supplemental_node.name }}"
+          virsh destroy "{{ supplemental_node.name|default('') }}"
         when: supplemental_check|success
         ignore_errors: true
 
       - name: Undefine supplemental vm
         command: >
-          virsh undefine "{{ supplemental_node.name }}" --remove-all-storage
+          virsh undefine "{{ supplemental_node.name|default('') }}" --remove-all-storage
         when: supplemental_check|success
         ignore_errors: true