Provide options to uninstall and stop Trento (Server and Agent)

[lee-martin]

During recent tests we had the need the both stop and uninstall Trento, but neither are formally implemented yet.

As workarounds we used:

• "systemctl stop k3s" to perform a stop
• "KUBECONFIG=~/.kube/config helm uninstall trento-server"

(which does not imply this is the best way to actually implement stop or uninstall :) )

[stefanotorresi]

We don't need to run "KUBECONFIG=~/.kube/config helm uninstall trento-server": as the Trento Server is completely contained in the K3S cluster, we just need to instruct how to stop/remove K3S.