Right now, `_, ok := provider.(Notifier); !ok` can mean one of two
things:
1. The provider does not support notification because the provided
content is static.
2. The implementor of the provider hasn't gotten around to implementing
Notifier yet.
These have very different implications. We should not force consumers of
these interfaces to have to figure out the static of Notifier across
sometimes numerous different implementations. Instead, we should force
implementors to implement Notifier, even if it's a noop.
Change-Id: Ie7a26697a9a17790bfaa58d67045663bcc71e3cb
Kubernetes-commit: 9b7d654a08d694d20226609f7075b112fb18639b
a6a121887a