Make sure the webhook is always registered before allowing user to create resources

[vdemeester]

As of today, knative/pkg's code is the one responsible to register the webhook on kubernetes. This means, the following are possible :

• The webhook fails to start (for any reason), it does not get registered, thus it's not applied
• The webhook takes time to start (slow network, …), it's not registered quickly enough, a user can create resources

In all those cases, the user can end up creating resources that are either not valid or not mutated (with defaults values)

There is two way to fix these cases:

1. Move the webhook registration in the yamls, so that the webhook is registered as soon as the CRDs are created, before it started, and thus it will fail if the webhook is not running
2. Having the controller validate

I have a very very high preference for solution 1. .

[vdemeester]

/assign

[bobcatfish]

@vdemeester are you still working on this or should we throw it in the backlog instead?