kvstoremesh: automatically propagate the cluster config from source

kvstoremesh: automatically propagate the cluster config from source

Slightly rework the logic constructing the CiliumClusterConfig object
propagated by KVStoreMesh, copying the one obtained from the source
cluster and explicitly overriding the fields that need to be overridden.
The benefits are two fold: (a) it makes the behavior more explicit,
highlighting which fields are simply propagated and which are actually
mutated, and (b) it avoids the need for tweaking it in case new fields
or capabilities are added in the future, which could be easily missed.

Signed-off-by: Marco Iorio <marco.iorio@isovalent.com>