Use ServiceInstance name as default value for CLUSTER_NAME

The following discussion from !8 (merged) should be addressed:

• @alossent started a discussion:

  The default isn't very useful here.

  I suggest to not require this parameter, and when not provided use the name of the ServiceInstance (which can be set to autogenerate names like swarm-ci-runners-12abc similar to pods names etc)

  Note the serviceInstance name is not readily available, we only have parameter _apb_service_instance_id. One needs to look in the target namespace for ServiceInstance objects whose .spec.externalID matches the value of _apb_service_instance_id