libvirt-metal bootstrap / 'cluster' unit needs to depend on 'libvirt-metal' unit

In this libvirt-metal CI job I noticed that the cluster unit tried to reconcile before the libvirt metal pods were up.

Indeed, we have been missing a dependency declaration to prevent that.

I noticed this issue on a run impacted by #1001 (closed), we had never ran into this missing dependency minor issue before probably because the libvirt-metal pods start up very quickly.