log-inventory: add missing zuul_info_dir prep

The role can't be used as-is without zuul_info_dir fact.

Change-Id: Id785f1b10e60dd09342edcebcbe7923b161add3c

roles/log-inventory/README.rst

@@ -1,3 +1,10 @@
 Log the inventory used to run the job to the job's log dir.
 
 This will result in the log collection roles logging the job inventory.
+
+**Role Variables**
+
+.. zuul:rolevar:: zuul_info_dir
+   :default: {{ zuul.executor.log_root }}/zuul-info
+
+   The directory path to store the inventory file.

roles/log-inventory/defaults/main.yaml

@@ -0,0 +1,2 @@
+---
+zuul_info_dir: "{{ zuul.executor.log_root }}/zuul-info"

roles/log-inventory/tasks/main.yaml

@@ -1,3 +1,10 @@
+- name: Ensure Zuul Ansible directory exists
+  delegate_to: localhost
+  run_once: true
+  file:
+    path: "{{ zuul_info_dir }}"
+    state: directory
+
 - name: Copy ansible inventory to logs dir
   delegate_to: localhost
   copy: