Ignore missing tox log directories

ironic-inspector redefines tox envdir, so the code to find the log files
from the environment fails. Add in an ignore to rsync so that files
being missing is not fatal.

Change-Id: Ie93af3569c13c57714e15bb6a9e852e44742662d

roles/fetch-tox-output/tasks/main.yaml

@@ -37,4 +37,6 @@
     mode: pull
     src: "{{ zuul_work_dir }}/.tox/{{ item }}/log/"
     verify_host: true
+    rsync_opts:
+      - "--ignore-missing-args"
   with_items: "{{ envlist }}"