Use sylva-units-operator to manage sylva-units helmrelease

Summary

Use sylva-units-operator to manage sylva-units helmrelease instead of using kustomizations

Several adaptations needs to be done:

• Install sylva-units operator early in bootstrap cluster (this should be done by sylvactl ultimately, but we'll probably use sylva-toolbox initially to shorten the release chain)
• Install it in management cluster befor pivot, and use it for pivot
• Adapt current environment-values/kustomizations to create sylva-units-release instead of the helmrelease