Bug 2175: K8s-Cluster Update is not implemented

Fix: handle (as agreed) the edit operation.
The update operation does not perform any operation on the K8SCluster.

Change-Id: I8ea45b1479608cdcae2140f1aa3b4cf541b2e643
Signed-off-by: dariofaccin <dario.faccin@canonical.com>
(cherry picked from commit 8bbeeb07)