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/
(cherry picked from commit 994c97e0bf)

Change-Id: Ie147257f6ed12617c4ab04f3e2d4d6735551ee57
Signed-off-by: Sagi Shnaidman <sshnaidm@redhat.com>

playbooks/devstack/post.yaml

@@ -1,4 +1,5 @@
 - hosts: all
   roles:
     - fetch-tox-output
-    - fetch-subunit-output
+    - role: fetch-subunit-output
+      when: fetch_subunit|default(true)|bool