Allow defining target namespace for the reference service instance

[Krzysztof Kwiatosz]

Description

Provide a --namespace option for the reference-instance command (or on a higher level) so that specifying target namespace for the k8s resources would be possible

Reasons

I cannot create reference service instance in a chosen namespace. CLI command always creates it in the default namespace.

./kyma-cli reference-instance --reference-name kk-object-store --name-selector=kk-object-store --offering-name=objectstore --namespace=kyma-system
unknown flag: --namespace

Use case: I want to create a reference-instnace (of an object store service) in kyma-system namespace so that the binding secret could be used in the kyma-system namespace (as s3-storage for docker-registry module installed in kyma-system namespace)