Automatic merge from submit-queue
shared controller informers
Related to https://github.com/kubernetes/kubernetes/issues/14978
This demonstrates how controllers which use an `Informer`, would be able to share the same watch and store. A similar "setup and run" approach could be done for an `IndexInformer` to share that cache. I found adding listeners here to be easier than intercepting at the watch interface (problems with resourceVersion) or the reflector (same plumbing, but you have to fan out to multiple stores).
We could also use the cache we build here to back several of the admission plugins that currently run their own lookup caches today.
If there's interest, I can finish out the `SharedInformer` and switch the low hanging fruit over.
@kubernetes/rh-cluster-infra @smarterclayton @liggitt @wojtek-t
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