RFE: More control over the environments

[vfarcic]

Given that environment pipelines are creating namespaces (e.g., jx-staging) there should be a mechanism to extend the definition by, for example, adding labels. A good use case is Istio which requires istio-enabled=true label. Without such a mechanism, users need to modify the namespaces manually and break the everything-in-git GitOps principle.