Run fetch-subunit-output role conditionally

Jobs that inherit from base job don't always run unit tests or
use subunit. This post playbook will fail for them, so make subinit
role optional and set by default for true.
For example disable it for ansible collection functional jobs:
https://review.opendev.org/#/c/711471/
Change-Id: I50b1020c896d7d8a7d58e62de778bb8c2b6e970e

playbooks/devstack/post.yaml

@@ -1,5 +1,6 @@
 - hosts: all
   roles:
     - fetch-tox-output
-    - fetch-subunit-output
+    - role: fetch-subunit-output
+      when: fetch_subunit|default(true)|bool
     - process-stackviz