Add label selector to deployment informer in gateway/deploymentcontroller.go

In gateway/deploymentcontroller.go we removed an optimization from the deployment informer (upstream uses a label selector to filter out just those deployments that are related to a gateway). We should find a way to add the label selector to a xns-informer-based informer.

Note: it's also possible that upstream will eventually remove this selector, at which point this Jira issue becomes moot.