Use Zuul v3 fetch-subunit-output

We have consolidated the fetch output roles into one
fetch-subunit-output, replace useage of old roles with new one.

Depends-On: I0cdfc66ee8b046affeb0b071fef38c21cb7a4948
Change-Id: I6d1726aa562f05b6d3bee7d0390fc1cf7cef4cd7

playbooks/devstack/post.yaml

@@ -1,4 +1,4 @@
 - hosts: all
   roles:
     - fetch-tox-output
-    - fetch-stestr-output
+    - fetch-subunit-output