Preparing group of host is not a "change"

This task is marked as "changed" while no change is really
happening on the nodes. We should mark this task as "unchanged"
as this is not a trigger of any conditional, and is confusing
in terms of idempotency.

Change-Id: I085050a33e7ce7dba9012f3b7c305c0290197f4b

playbooks/repo-build.yml

@@ -38,6 +38,7 @@
         name: "{{ groups[item][0] }}"
         groups: repo_masters
       with_items: "{{ groups | select('match', '^repo_servers_') | list }}"
+      changed_when: false
   tags:
     - always
     - repo-build