Do not install Velero in CI

After some discussion in !292 (merged) , we decided to not install Velero in the regular CI runs. However I failed to execute that change properly: the ansible tasks file velero.yml is not run when Velero is disabled, but that means that among other things the velero-settings secret is not created, and right now our flux config (in /flux/.flux.yaml defaults to enable an app if it can't find the corresponding settings secret, so Velero is installed after all.

I propose to change that default, so apps that do not have a settings secret are not installed. This doesn't matter as long as the cluster is managed through our playbook, because that will create the secrets, but it does make it slightly harder to add a custom app to OpenAppStack: you have to create this settings secret besides having a HelmRelease file in you flux folder.