apply scripts: wait for root-dependency to be ready before looking at all units

In the context of #1852 (closed) and !3326 (merged) we realized with @feleouet that we will need to wait for the root-dependency Kustomization to be ready before using sylvactl watch on the sylva-units-status Kustomization. If we don't do this there are possible race where it may see a mix of old Kustomizations and new ones and either display garbage or (worse) get to wrong conclusions on the state of the system.