log-inventory: set inventory_file if needed

Job running without nodeset doesn't have the inventory_file.

Depends-On: https://review.openstack.org/578235
Change-Id: I5d543b4e30746a64d9bd7706ffd52d41e3710ee6

roles/log-inventory/tasks/main.yaml

@@ -1,3 +1,8 @@
+- name: Set inventory_file fact
+  set_fact:
+    inventory_file: "{{ zuul.executor.inventory_file }}"
+  when: inventory_file is not defined
+
 - name: Ensure Zuul Ansible directory exists
   delegate_to: localhost
   run_once: true