[tests] Improve devstack/post playbook efficiency

By adjusting the syntax to the newer format, the fetch-subunit
role will be skipped entirely, rather than run with all its
tasks skipped. This gives us results sooner and burns less
electricity, making the world a better place. :)

Change-Id: I6ae5a63051d512eecb2a603af9d74ec139eb0877

playbooks/devstack/post.yaml

@@ -1,6 +1,9 @@
 - hosts: all
-  roles:
+  tasks:
-    - fetch-tox-output
+    - include_role:
-    - role: fetch-subunit-output
+        name: fetch-tox-output
+    - include_role:
+        name: fetch-subunit-output
       when: fetch_subunit|default(true)|bool
-    - process-stackviz
+    - include_role:
+        name: process-stackviz