Bug 2174: VCA Update is not implemented

Fix: Handle the edit operation.
If no update on the VCA configuration is given as input, then the operation is
marked as completed.
Otherwise the validation of the new VCA configuration is performed,
which may cause the edit operation to fail.

Change-Id: Id13604664df3594874877d0cb2ad13b1ce6d2353
Signed-off-by: Dario Faccin <dario.faccin@canonical.com>