Rename waiter-cluster K8s label to waiter/cluster for consistency

[DaoWen]

For Kubernetes objects created by Waiter, we should be using the namespace/property naming convention for all of our Waiter-specific annotations and labels.

This change would be two stages:

1. Add the waiter/cluster label redundantly.
2. After some reasonable amount of time (i.e., when we're confident that all of the objects without waiter/cluster have been deleted or replaced), remove the waiter-cluster label and switch our queries (e.g., watches) to use the new waiter/cluster label.