Split logging of inventory to its own role

The ability to read and write the inventory is privileged and requires
you are running in a trusted context. This makes testing the
validate-host role difficult as it reads the inventory file and writes
it to the job log dir.

Make a new role called log-inventory that will allow us to eventually
remove this task from validate-host enabling better testing of validate
host. Also this will allow us to log the inventory in the post-run
logging activity.

Change-Id: I4098fd140484bc54dc95b5f4c51afb1c56c7b19c

roles/log-inventory/README.rst

@@ -0,0 +1,3 @@
+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.

roles/log-inventory/tasks/main.yaml

@@ -0,0 +1,5 @@
+- name: Copy ansible inventory to logs dir
+  delegate_to: localhost
+  copy:
+    src: "{{ inventory_file }}"
+    dest: "{{ zuul_info_dir }}"