Allow operators to control feature gates

[bsnchan]

https://kubernetes.io/docs/reference/feature-gates/

/cc @dprotaso

[cfdreddbot]

Hey bsnchan!

Thanks for submitting this pull request! I'm here to inform the recipients of the pull request that you and the commit authors have already signed the CLA.

[cf-gitbot]

We have created an issue in Pivotal Tracker to manage this:

https://www.pivotaltracker.com/story/show/157161612

The labels on this github issue will be updated when the story is started.

[tvs]

This can be very useful. I suppose it's possible that the operator could use this to disable a feature that defaults to on and that we rely on, but given that there are so many other ways to break the cluster (e.g. configuring wonky certs), I'm not sure that matters.

[bsnchan]

@glestaris any thoughts about this? Some of the features we wanted to try out are CustomResourceValidation and CustomResourceSubresources.