provide sane default value for cluster_domain of workload clusters

Today we don't have any setting for cluster_domain that would be specific to a workload cluster.

As a result, by default, workload cluster will use sylva, which means that if the mgmt cluster also does the same, the mgmt cluster and all the workload clusters will get the same hostname for a service that they would all expose, for instance Harbor: harbor.sylva.

In practice all those names would typicall differ and be properly tuned.

This MR does two things:

• ensure that the default setting does not mislead people would read/maintain the code into thinking that sylva is an appropriate setting for workload clusters
• have a sane default for the domain of a workload cluster, derived from the one set on the mgmt cluster: <workload-cluster-name>.wclusters.<mgmt-cluster-domain>.