Do not fail even if dnf cleanup fails

Since ansible 2.8, dnf module fails if repository name contains '*' and
no repogitory which matches the expression detected.
This patch makes current playbook avoid errors during dnf cleanup task,
so that the tasks can pass even if there is no dnf repositories
configured.

Change-Id: Ie0a7e6455e68b387a9d392fc1fc8c970da86cddf

playbooks/prepare-node-common.yaml

@@ -58,6 +58,7 @@
           name: "{{ item }}"
           state: absent
         become: true
+        ignore_errors: true
         with_items:
           - rdo-release
           - centos-release-openstack-*